enhancement: adopt Swift 6 language mode and make the global hot-path state concurrency-safe

[kazukinakai]

Problem

Package.swift is swift-tools-version: 5.10 with no Swift 6 language mode. The CGEvent-tap hot path reads module-level var globals — shortcutList, exclusionAppsDict, isRecordingShortcut, keyMappingList, activeAppsList, exclusionAppsList.

Today there is no live data race: the tap's run-loop source is added to the main run loop (CFRunLoopGetMain()), and AppSettings (the writer) is @MainActor — so every reader and writer is on the main thread. But that contract is invisible to the compiler. Under Swift 6 strict concurrency these globals are hard errors (non-Sendable mutable global state), and the safety relies entirely on a convention that is easy to break (e.g. moving the tap to a background run loop).

Suggested approach

1. Wrap the shared caches in a @MainActor holder type (or a small lock-guarded struct).
2. Enable Swift 6 language mode (swiftLanguageModes: [.v6] / per-target setting).
3. Let the compiler enforce the main-thread contract that is currently only documented in CLAUDE.md.

Forward-looking; related to closed #73 ("crash risk under Swift 6").

[kazukinakai]

Triage (2026-06-08 audit): still valid and correctly the only audit issue left open. Verified current state — Package.swift is swift-tools-version: 5.10 (not Swift 6 mode), and the hot-path globals shortcutList / exclusionAppsDict (KeyMappings.swift / KeyEvent.swift) remain module-level mutable vars read on the CGEvent-tap callback thread while written on main. This is the genuine remaining concurrency-safety work; #81–#94 are all resolved and now closed.