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.

1 files changed, 5 insertions, 4 deletions

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>