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, 4 insertions, 5 deletions

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)
     }
 }