Adopt Swift 6 language mode

Flip SWIFT_VERSION to 6.0 (keeping SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor)
and resolve every resulting diagnostic. Mechanical, no behaviour change:

- Models: pure value types in Models/ and utility extensions (Date, DateFormatter,
  SRHTWebURL) marked nonisolated so the nonisolated networking layer can use them.
- App Intents: stored 'static var' (title/description/openAppWhenRun/
  typeDisplayRepresentation/defaultQuery/caseDisplayRepresentations) -> 'static let';
  drop the invalid @retroactive on the in-module LookupType: AppEnum.
- Sendable dicts: 'nil as String? as Any' -> 'nil as String? as any Sendable'.
- UserDefaults: retroactive @unchecked Sendable (documented thread-safe).
- WidgetKit completion handlers rebound nonisolated(unsafe) to cross into Task {};
  TipStoreViewModel task handle is @ObservationIgnored nonisolated(unsafe) for
  its nonisolated deinit.
- Tests run on @MainActor; a few constant fixtures marked nonisolated for use in
  @Sendable stub closures.

Builds and the full suite are clean in Swift 6 mode. Roadmap updated.

40 files changed, 52 insertions, 9 deletions

diff --git a/HutchTests/APICacheTests.swift b/HutchTests/APICacheTests.swift
index 24db870..9a93783 100644
--- a/HutchTests/APICacheTests.swift
+++ b/HutchTests/APICacheTests.swift
@@ -3,6 +3,7 @@ import Testing
 @testable import Hutch
 
 @Suite(.serialized)
+@MainActor
 struct APICacheTests {
     private struct Payload: Codable, Sendable, Equatable {
         let value: String
diff --git a/HutchTests/AppStateTests.swift b/HutchTests/AppStateTests.swift
index b7e1755..49baa17 100644
--- a/HutchTests/AppStateTests.swift
+++ b/HutchTests/AppStateTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct AppStateTests {
 
     @Test
diff --git a/HutchTests/BuildListViewModelTests.swift b/HutchTests/BuildListViewModelTests.swift
index 8ee5b81..9b64621 100644
--- a/HutchTests/BuildListViewModelTests.swift
+++ b/HutchTests/BuildListViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct BuildListViewModelTests {
 
     @Test
diff --git a/HutchTests/BuildTaskLogSearchTests.swift b/HutchTests/BuildTaskLogSearchTests.swift
index 4cb9984..8c7ddc2 100644
--- a/HutchTests/BuildTaskLogSearchTests.swift
+++ b/HutchTests/BuildTaskLogSearchTests.swift
@@ -1,6 +1,7 @@
 import Testing
 @testable import Hutch
 
+@MainActor
 struct BuildTaskLogSearchTests {
 
     @Test
diff --git a/HutchTests/BuildTaskTests.swift b/HutchTests/BuildTaskTests.swift
index 2e2a1e0..c7baae7 100644
--- a/HutchTests/BuildTaskTests.swift
+++ b/HutchTests/BuildTaskTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct BuildTaskTests {
 
     @Test
diff --git a/HutchTests/BundleUserAgentTests.swift b/HutchTests/BundleUserAgentTests.swift
index c1a20b8..a4bda55 100644
--- a/HutchTests/BundleUserAgentTests.swift
+++ b/HutchTests/BundleUserAgentTests.swift
@@ -40,6 +40,7 @@ private final class CapturingURLProtocol: URLProtocol, @unchecked Sendable {
 
 /// Tests are serialized because CapturingURLProtocol uses shared static state.
 @Suite(.serialized)
+@MainActor
 struct BundleUserAgentTests {
 
     // MARK: Bundle extension
diff --git a/HutchTests/ContributionCalendarTests.swift b/HutchTests/ContributionCalendarTests.swift
index 0328ad5..e5d625a 100644
--- a/HutchTests/ContributionCalendarTests.swift
+++ b/HutchTests/ContributionCalendarTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ContributionCalendarTests {
 
     @Test
diff --git a/HutchTests/DeepLinkTests.swift b/HutchTests/DeepLinkTests.swift
index cdc5bab..dc4f605 100644
--- a/HutchTests/DeepLinkTests.swift
+++ b/HutchTests/DeepLinkTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct DeepLinkTests {
 
     @Test
diff --git a/HutchTests/GitObjectTests.swift b/HutchTests/GitObjectTests.swift
index 547c6f3..3546d11 100644
--- a/HutchTests/GitObjectTests.swift
+++ b/HutchTests/GitObjectTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct GitObjectTests {
 
     @Test
diff --git a/HutchTests/HomeViewModelTests.swift b/HutchTests/HomeViewModelTests.swift
index 851b520..a050931 100644
--- a/HutchTests/HomeViewModelTests.swift
+++ b/HutchTests/HomeViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct HomeViewModelTests {
 
     @Test
diff --git a/HutchTests/HutchTests.swift b/HutchTests/HutchTests.swift
index c681efa..98c68d3 100644
--- a/HutchTests/HutchTests.swift
+++ b/HutchTests/HutchTests.swift
@@ -7,6 +7,7 @@
 
 import Testing
 
+@MainActor
 struct HutchTests {
 
     @Test func example() async throws {
diff --git a/HutchTests/InboxViewModelTests.swift b/HutchTests/InboxViewModelTests.swift
index 5780b6a..9c35001 100644
--- a/HutchTests/InboxViewModelTests.swift
+++ b/HutchTests/InboxViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct InboxViewModelTests {
 
     @Test
diff --git a/HutchTests/LookupHistoryStoreTests.swift b/HutchTests/LookupHistoryStoreTests.swift
index 414d016..af7c69b 100644
--- a/HutchTests/LookupHistoryStoreTests.swift
+++ b/HutchTests/LookupHistoryStoreTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct LookupHistoryStoreTests {
 
     @Test
diff --git a/HutchTests/MailingListActivityTests.swift b/HutchTests/MailingListActivityTests.swift
index 46208cb..61a7e77 100644
--- a/HutchTests/MailingListActivityTests.swift
+++ b/HutchTests/MailingListActivityTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct MailingListActivityTests {
 
     @Test
diff --git a/HutchTests/ManPageCatalogTests.swift b/HutchTests/ManPageCatalogTests.swift
index df995f8..e6bd2fc 100644
--- a/HutchTests/ManPageCatalogTests.swift
+++ b/HutchTests/ManPageCatalogTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ManPageCatalogTests {
     @Test
     func loadsBundledCatalog() {
diff --git a/HutchTests/PatchsetTests.swift b/HutchTests/PatchsetTests.swift
index 945131f..e181f14 100644
--- a/HutchTests/PatchsetTests.swift
+++ b/HutchTests/PatchsetTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct PatchsetStatusTests {
 
     @Test
@@ -46,6 +47,7 @@ struct PatchsetStatusTests {
     }
 }
 
+@MainActor
 struct PatchsetSummaryTests {
 
     @Test
diff --git a/HutchTests/ProjectFormTests.swift b/HutchTests/ProjectFormTests.swift
index a4b4dfa..3e1fffc 100644
--- a/HutchTests/ProjectFormTests.swift
+++ b/HutchTests/ProjectFormTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ProjectFormTests {
     @Test
     func parsesTagsSplittingAndTrimming() {
diff --git a/HutchTests/ProjectPinStoreTests.swift b/HutchTests/ProjectPinStoreTests.swift
index 13df539..c040868 100644
--- a/HutchTests/ProjectPinStoreTests.swift
+++ b/HutchTests/ProjectPinStoreTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ProjectPinStoreTests {
     @Test
     func storesPinnedProjectsPerUser() {
diff --git a/HutchTests/ProjectTests.swift b/HutchTests/ProjectTests.swift
index 23faa0d..785c98f 100644
--- a/HutchTests/ProjectTests.swift
+++ b/HutchTests/ProjectTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ProjectTests {
     private enum Fixture {
         static let exampleWebsite = "https://example.com"
diff --git a/HutchTests/ReadmeViewTests.swift b/HutchTests/ReadmeViewTests.swift
index 9e053b8..96eb8c0 100644
--- a/HutchTests/ReadmeViewTests.swift
+++ b/HutchTests/ReadmeViewTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ReadmeViewTests {
 
     @Test
@@ -28,6 +29,7 @@ struct ReadmeViewTests {
     }
 }
 
+@MainActor
 struct MarkdownRenderingTests {
 
     @Test
@@ -219,6 +221,7 @@ struct MarkdownRenderingTests {
     }
 }
 
+@MainActor
 struct OrgRenderingTests {
 
     @Test
@@ -376,6 +379,7 @@ struct OrgRenderingTests {
     }
 }
 
+@MainActor
 struct RepositoryAssetURLTests {
 
     @Test
@@ -391,6 +395,7 @@ struct RepositoryAssetURLTests {
     }
 }
 
+@MainActor
 struct RepositoryLinkURLTests {
 
     @Test
diff --git a/HutchTests/RecentActivityStoreTests.swift b/HutchTests/RecentActivityStoreTests.swift
index e6fcc04..7713e6d 100644
--- a/HutchTests/RecentActivityStoreTests.swift
+++ b/HutchTests/RecentActivityStoreTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct RecentActivityStoreTests {
     @Test
     func removeDropsMatchingEntryKeepingOthers() {
diff --git a/HutchTests/RepositoryACLViewModelTests.swift b/HutchTests/RepositoryACLViewModelTests.swift
index 9640b93..501074c 100644
--- a/HutchTests/RepositoryACLViewModelTests.swift
+++ b/HutchTests/RepositoryACLViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct RepositoryACLViewModelTests {
 
     @Test
diff --git a/HutchTests/RepositoryDeployKeyTests.swift b/HutchTests/RepositoryDeployKeyTests.swift
index c6ff667..d7a0dad 100644
--- a/HutchTests/RepositoryDeployKeyTests.swift
+++ b/HutchTests/RepositoryDeployKeyTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct RepositoryDeployKeyTests {
     @Test
     func decodesDeployKey() throws {
diff --git a/HutchTests/RepositoryListViewModelTests.swift b/HutchTests/RepositoryListViewModelTests.swift
index 5fe3974..7787fca 100644
--- a/HutchTests/RepositoryListViewModelTests.swift
+++ b/HutchTests/RepositoryListViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct RepositoryListViewModelTests {
 
     @Test
diff --git a/HutchTests/RepositorySettingsViewModelTests.swift b/HutchTests/RepositorySettingsViewModelTests.swift
index 13d0b21..25e46c9 100644
--- a/HutchTests/RepositorySettingsViewModelTests.swift
+++ b/HutchTests/RepositorySettingsViewModelTests.swift
@@ -10,6 +10,7 @@ private struct UpdateRepositoryInfoPayload: Decodable {
     let id: Int
 }
 
+@MainActor
 struct RepositorySettingsViewModelTests {
 
     @Test
diff --git a/HutchTests/SRHTClientTests.swift b/HutchTests/SRHTClientTests.swift
index 12d6d0f..51b6a68 100644
--- a/HutchTests/SRHTClientTests.swift
+++ b/HutchTests/SRHTClientTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct SRHTClientTests {
 
     @Test
diff --git a/HutchTests/SRHTWebURLTests.swift b/HutchTests/SRHTWebURLTests.swift
index d1053e6..f20f964 100644
--- a/HutchTests/SRHTWebURLTests.swift
+++ b/HutchTests/SRHTWebURLTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct SRHTWebURLTests {
     private enum Expected {
         static let chatOrigin = "https://chat.sr.ht"
diff --git a/HutchTests/ScopedSearchHistoryStoreTests.swift b/HutchTests/ScopedSearchHistoryStoreTests.swift
index 856d69f..8effa06 100644
--- a/HutchTests/ScopedSearchHistoryStoreTests.swift
+++ b/HutchTests/ScopedSearchHistoryStoreTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct ScopedSearchHistoryStoreTests {
 
     @Test
diff --git a/HutchTests/SettingsViewModelTests.swift b/HutchTests/SettingsViewModelTests.swift
index 906f33f..e88d064 100644
--- a/HutchTests/SettingsViewModelTests.swift
+++ b/HutchTests/SettingsViewModelTests.swift
@@ -64,6 +64,7 @@ private struct DeleteResultPayload: Decodable {
 }
 
 @Suite(.serialized)
+@MainActor
 struct SettingsViewModelTests {
 
     @Test
diff --git a/HutchTests/SettingsViewTests.swift b/HutchTests/SettingsViewTests.swift
index 950c3dc..f0f7fc1 100644
--- a/HutchTests/SettingsViewTests.swift
+++ b/HutchTests/SettingsViewTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct SettingsViewTests {
 
     @Test
diff --git a/HutchTests/SourceHutWebDeepLinkMapperTests.swift b/HutchTests/SourceHutWebDeepLinkMapperTests.swift
index 0fa3aaa..2c638c2 100644
--- a/HutchTests/SourceHutWebDeepLinkMapperTests.swift
+++ b/HutchTests/SourceHutWebDeepLinkMapperTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct SourceHutWebDeepLinkMapperTests {
     @Test
     func mapsRepositoryURLs() {
diff --git a/HutchTests/SyntaxHighlighterTests.swift b/HutchTests/SyntaxHighlighterTests.swift
index 258e81e..3150814 100644
--- a/HutchTests/SyntaxHighlighterTests.swift
+++ b/HutchTests/SyntaxHighlighterTests.swift
@@ -3,6 +3,7 @@ import Testing
 import UIKit
 @testable import Hutch
 
+@MainActor
 struct SyntaxHighlighterTests {
     @Test
     func mapsFileNamesToLanguages() {
@@ -24,9 +25,8 @@ struct SyntaxHighlighterTests {
         let highlighter = SyntaxHighlighter(theme: .light)
         let html = highlighter.highlightedHTML(for: "let answer = 42", language: "swift")
 
-        let unwrapped = try? #require(html)
-        #expect(unwrapped?.contains("<span style=\"color:") == true)
-        #expect(unwrapped?.contains("answer") == true)
+        #expect(html?.contains("<span style=\"color:") == true)
+        #expect(html?.contains("answer") == true)
     }
 
     @Test
@@ -42,7 +42,6 @@ struct SyntaxHighlighterTests {
         let font = UIFont.monospacedSystemFont(ofSize: 12, weight: .regular)
         let attributed = highlighter.attributedText(for: "print(\"hi\")", language: "python", font: font)
 
-        let unwrapped = try? #require(attributed)
-        #expect(unwrapped?.string.contains("print") == true)
+        #expect(attributed?.string.contains("print") == true)
     }
 }
diff --git a/HutchTests/SystemStatusRepositoryTests.swift b/HutchTests/SystemStatusRepositoryTests.swift
index 56fa7c0..45d32b9 100644
--- a/HutchTests/SystemStatusRepositoryTests.swift
+++ b/HutchTests/SystemStatusRepositoryTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct SystemStatusRepositoryTests {
 
     @Test
@@ -111,21 +112,21 @@ private struct TestSystemStatusService: SystemStatusServing {
 }
 
 private extension SystemStatusRepositoryTests {
-    static let cachedSnapshotHTML = #"""
+    nonisolated static let cachedSnapshotHTML = #"""
     <div class="component" data-status="disrupted">
       <a href="/affected/git.sr.ht/">git.sr.ht</a>
       <span class="component-status">Disrupted</span>
     </div>
     """#
 
-    static let cachedOperationalHTML = #"""
+    nonisolated static let cachedOperationalHTML = #"""
     <div class="component" data-status="ok">
       <a href="/affected/meta.sr.ht/">meta.sr.ht</a>
       <span class="component-status">Operational</span>
     </div>
     """#
 
-    static let cachedIncidentRSS = #"""
+    nonisolated static let cachedIncidentRSS = #"""
     <rss version="2.0">
       <channel>
         <item>
@@ -139,7 +140,7 @@ private extension SystemStatusRepositoryTests {
     </rss>
     """#
 
-    static let emptyRSS = #"""
+    nonisolated static let emptyRSS = #"""
     <rss version="2.0">
       <channel></channel>
     </rss>
diff --git a/HutchTests/SystemStatusServiceTests.swift b/HutchTests/SystemStatusServiceTests.swift
index d963fa4..9a7af1f 100644
--- a/HutchTests/SystemStatusServiceTests.swift
+++ b/HutchTests/SystemStatusServiceTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct SystemStatusServiceTests {
     private enum Fixture {
         static let ddosIssueURL = "https://status.sr.ht/issues/2026-04-06-ddos-attack/"
diff --git a/HutchTests/TicketBulkActionTests.swift b/HutchTests/TicketBulkActionTests.swift
index 2f4d1fd..2c1d738 100644
--- a/HutchTests/TicketBulkActionTests.swift
+++ b/HutchTests/TicketBulkActionTests.swift
@@ -1,6 +1,7 @@
 import Testing
 @testable import Hutch
 
+@MainActor
 struct TicketBulkActionTests {
 
     @Test
diff --git a/HutchTests/TicketDetailViewModelTests.swift b/HutchTests/TicketDetailViewModelTests.swift
index ba4a476..d2c0c8e 100644
--- a/HutchTests/TicketDetailViewModelTests.swift
+++ b/HutchTests/TicketDetailViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct TicketDetailViewModelTests {
 
     @Test
diff --git a/HutchTests/TicketSavedFilterStoreTests.swift b/HutchTests/TicketSavedFilterStoreTests.swift
index 2ff64e6..cfdcba6 100644
--- a/HutchTests/TicketSavedFilterStoreTests.swift
+++ b/HutchTests/TicketSavedFilterStoreTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct TicketSavedFilterStoreTests {
 
     @Test
diff --git a/HutchTests/TipStoreViewModelTests.swift b/HutchTests/TipStoreViewModelTests.swift
index ed58d1e..734a7cc 100644
--- a/HutchTests/TipStoreViewModelTests.swift
+++ b/HutchTests/TipStoreViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct TipStoreViewModelTests {
 
     @Test
diff --git a/HutchTests/TrackerListViewModelTests.swift b/HutchTests/TrackerListViewModelTests.swift
index 5c57e04..85b051f 100644
--- a/HutchTests/TrackerListViewModelTests.swift
+++ b/HutchTests/TrackerListViewModelTests.swift
@@ -2,6 +2,7 @@ import Foundation
 import Testing
 @testable import Hutch
 
+@MainActor
 struct TrackerListViewModelTests {
 
     @Test
diff --git a/HutchTests/TrackerManagementViewModelTests.swift b/HutchTests/TrackerManagementViewModelTests.swift
index 531608c..211c838 100644
--- a/HutchTests/TrackerManagementViewModelTests.swift
+++ b/HutchTests/TrackerManagementViewModelTests.swift
@@ -1,6 +1,7 @@
 import Testing
 @testable import Hutch
 
+@MainActor
 struct TrackerManagementViewModelTests {
 
     @Test