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, 12 insertions, 12 deletions

diff --git a/Hutch/Models/Project.swift b/Hutch/Models/Project.swift
index 8f5aa68..bd1a688 100644
--- a/Hutch/Models/Project.swift
+++ b/Hutch/Models/Project.swift
@@ -1,7 +1,7 @@
 import Foundation
 
-struct Project: Identifiable, Hashable, Sendable {
-    struct MailingList: Identifiable, Hashable, Sendable {
+nonisolated struct Project: Identifiable, Hashable, Sendable {
+    nonisolated struct MailingList: Identifiable, Hashable, Sendable {
         let id: String
         let name: String
         let description: String?
@@ -22,8 +22,8 @@ struct Project: Identifiable, Hashable, Sendable {
         }
     }
 
-    struct SourceRepo: Identifiable, Hashable, Sendable {
-        enum RepoType: String, Decodable, Sendable {
+    nonisolated struct SourceRepo: Identifiable, Hashable, Sendable {
+        nonisolated enum RepoType: String, Decodable, Sendable {
             case git = "GIT"
             case hg = "HG"
 
@@ -47,7 +47,7 @@ struct Project: Identifiable, Hashable, Sendable {
         }
     }
 
-    struct Tracker: Identifiable, Hashable, Sendable {
+    nonisolated struct Tracker: Identifiable, Hashable, Sendable {
         let id: String
         let name: String
         let description: String?
@@ -72,7 +72,7 @@ struct Project: Identifiable, Hashable, Sendable {
     let isFullyLoaded: Bool
 
     /// Identity and display fields for a project.
-    struct Metadata: Sendable, Hashable {
+    nonisolated struct Metadata: Sendable, Hashable {
         let id: String
         let name: String
         let description: String?
@@ -82,7 +82,7 @@ struct Project: Identifiable, Hashable, Sendable {
         let updated: Date
     }
 
-    struct Resources: Sendable, Hashable {
+    nonisolated struct Resources: Sendable, Hashable {
         let mailingLists: [MailingList]
         let sources: [SourceRepo]
         let trackers: [Tracker]
@@ -171,7 +171,7 @@ struct Project: Identifiable, Hashable, Sendable {
     }
 }
 
-extension Project.MailingList {
+nonisolated extension Project.MailingList {
     var displayName: String {
         Project.normalizedText(name) ?? "Untitled Mailing List"
     }
@@ -185,7 +185,7 @@ extension Project.MailingList {
     }
 }
 
-extension Project.SourceRepo {
+nonisolated extension Project.SourceRepo {
     var displayName: String {
         Project.normalizedText(name) ?? "Untitled Repository"
     }
@@ -203,7 +203,7 @@ extension Project.SourceRepo {
     }
 }
 
-extension Project.Tracker {
+nonisolated extension Project.Tracker {
     var displayName: String {
         Project.normalizedText(name) ?? "Untitled Tracker"
     }
@@ -221,13 +221,13 @@ extension Project.Tracker {
     }
 }
 
-extension String {
+nonisolated extension String {
     var srhtUsername: String {
         hasPrefix("~") ? String(dropFirst()) : self
     }
 }
 
-extension Visibility {
+nonisolated extension Visibility {
     var displayName: String {
         switch self {
         case .publicVisibility: