feat(macos): add macOS menu-bar app and harden cross-device iCloud sync

Introduce a standalone macOS menu-bar app (OSGKeyboardMac) that reuses the
platform-agnostic OSGKeyboardShared core: record -> cloud/local ASR -> polish
-> insert. Local mode uses Qwen3-ASR via mlx-swift-asr (macOS 15+, Apple
Silicon); iOS targets stay zero-SPM.

Harden iCloud sync for multi-device correctness:
- Per-field settings merge (appSettings.v2) so concurrent edits no longer
  clobber each other's unrelated fields.
- Per-device usage statistics (G-Counter) that sum instead of max().
- Tombstoned dictionary/history merge so deletes propagate and entries can't
  resurrect.
- API keys replicate via iCloud Keychain, never iCloud KVS JSON; pulling a
  legacy blob without key fields no longer wipes local Keychain entries.
- Add a low-risk "Sync Now" action in Settings.

Fix Flow keyboard mic state: stay orange until the host publishes a real ready
contract, share a single MicVoiceAvailability gate, and self-heal stale
cross-process heartbeat jitter instead of getting stuck.

Extract shared storage (SpeechHistoryStore/UsageStatisticsStore,
ConfigurationStore) into OSGKeyboardShared and add tests for the new
sync/merge logic.

OSGKeyboardShared/Services/FlowSessionDarwin.swift

@@ -10,6 +10,8 @@ public enum FlowSessionDarwin {
     public static let notificationName = "com.osgkeyboard.flow.session.changed"
     /// Posted when the host app writes a transcription result or error.
     public static let transcriptionNotificationName = "com.osgkeyboard.flow.transcription.changed"
+    /// Posted when the host app publishes or clears the ready contract.
+    public static let hostReadyNotificationName = "com.osgkeyboard.flow.host.ready.changed"
 
     public static func postSessionChanged() {
         CFNotificationCenterPostNotification(
@@ -30,6 +32,16 @@ public enum FlowSessionDarwin {
             true
         )
     }
+
+    public static func postHostReadyChanged() {
+        CFNotificationCenterPostNotification(
+            CFNotificationCenterGetDarwinNotifyCenter(),
+            CFNotificationName(hostReadyNotificationName as CFString),
+            nil,
+            nil,
+            true
+        )
+    }
 }
 
 /// Observes Darwin notifications on a background thread; invokes