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/FlowSessionKeys.swift

@@ -10,6 +10,10 @@ public enum FlowSessionKeys {
     public static let flowSessionActive = "flow.flowSessionActive"
     public static let flowSessionExpires = "flow.flowSessionExpires"
     public static let flowHeartbeat = "flow.flowHeartbeat"
+    /// Host-published contract: capture + polling idle and able to accept utterances.
+    public static let flowHostReady = "flow.flowHostReady"
+    /// Wall-clock timestamp paired with `flowHostReady` (seconds since 1970).
+    public static let flowHostReadyAt = "flow.flowHostReadyAt"
     public static let keyboardRecordingState = "flow.keyboardRecordingState"
     public static let transcriptionLanguage = "flow.transcriptionLanguage"
     public static let transcriptionResult = "flow.transcriptionResult"
@@ -29,6 +33,9 @@ public enum FlowSessionKeys {
     /// Heartbeat older than this → host is not actively reachable for recording.
     public static let heartbeatStaleInterval: TimeInterval = 3
 
+    /// `flowHostReadyAt` must be within this window of the latest heartbeat.
+    public static let hostReadyMaxHeartbeatSkew: TimeInterval = 5
+
     /// Session flag still set but heartbeat older than this → host process is
     /// dead (force-quit, reboot). Keyboard / host should clear persisted state.
     public static let heartbeatZombieInterval: TimeInterval = 60