refactor(apple): adopt Swift 6 language mode

Enable complete strict concurrency and switch the app target to Swift 6.

- Isolate model dependency protocols to @MainActor
- Make the CoreRepository blocking-FFI bridge race-free: nonisolated(unsafe)
  core handle, nonisolated runCore/readSnapshot, @Sendable work block,
  Sendable domain models
- Fix Binding method-reference captures; @preconcurrency imports for
  CoreNFC/AVFoundation/VnidropCore; isolate the NFC/QR delegate helpers

apple/project.yml

@@ -37,7 +37,8 @@ targets:
         # (the editor reads these build settings, not the manual plist).
         INFOPLIST_KEY_CFBundleDisplayName: VniDrop
         INFOPLIST_KEY_LSApplicationCategoryType: public.app-category.utilities
-        SWIFT_VERSION: "5.9"
+        SWIFT_VERSION: "6.0"
+        SWIFT_STRICT_CONCURRENCY: complete
         ENABLE_USER_SCRIPT_SANDBOXING: NO
         ASSETCATALOG_COMPILER_APPICON_NAME: AppIcon
         CODE_SIGN_STYLE: Automatic