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.

OSGKeyboardExt/Services/KeyboardConfigSync.swift

@@ -18,6 +18,7 @@ final class KeyboardConfigSync {
 
     private var flowSessionDarwinObserver: FlowSessionDarwinObserver?
     private var transcriptionDarwinObserver: FlowSessionDarwinObserver?
+    private var hostReadyDarwinObserver: FlowSessionDarwinObserver?
     private var configDarwinObserver: FlowSessionDarwinObserver?
 
     init(
@@ -39,6 +40,11 @@ final class KeyboardConfigSync {
         ) { [weak self] in
             self?.onFlowSessionChanged()
         }
+        hostReadyDarwinObserver = FlowSessionDarwinObserver(
+            notificationName: FlowSessionDarwin.hostReadyNotificationName
+        ) { [weak self] in
+            self?.onFlowSessionChanged()
+        }
         configDarwinObserver = FlowSessionDarwinObserver(
             notificationName: AppGroupConfigDarwin.notificationName
         ) { [weak self] in