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, 0 deletions

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