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.

CHANGELOG.md

@@ -7,6 +7,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- **iCloud sync hardening**: per-field settings merge (`appSettings.v2`), per-device usage statistics (G-Counter), tombstoned dictionary/history merge, and a low-risk **Sync Now** action in Settings. / **iCloud 同步加固**：设置按字段合并（`appSettings.v2`）、统计按设备 G-Counter 累计、词库/历史带墓碑合并，并在设置中新增低风险的**立即同步**操作。
+
+### Changed
+- **API key sync**: cloud provider API keys now replicate through **iCloud Keychain** when settings sync is on — never through iCloud KVS JSON. / **API 密钥同步**：开启设置同步后，云端服务商 API 密钥改由 **iCloud 钥匙串**复制，不再写入 iCloud KVS JSON。
+- **Speech history cap**: synced history limit is **300** entries (aligned with the sync payload). / **语音历史上限**：可同步历史上限为 **300** 条（与同步载荷一致）。
+
+### Fixed
+- **Settings sync wiping API keys**: pulling a legacy settings blob without API key fields no longer deletes local Keychain entries. / **设置同步清空 API 密钥**：拉取不含 API 密钥字段的旧版设置包时，不再删除本地 Keychain 项。
+- **Cross-device settings conflicts**: changing different settings on two devices no longer lets one device's full blob overwrite the other's unrelated fields. / **跨设备设置冲突**：两台设备分别修改不同设置项时，不再因整包覆盖而冲掉对方未改动的字段。
+- **Usage statistics under-counting**: offline usage on multiple devices now sums correctly instead of taking per-field `max()`. / **使用统计少计**：多设备离线各自累计后合并为求和，不再对总量取 `max()`。
+- **Dictionary/history resurrection**: deletes and "clear all" on one device propagate via tombstones so older remote entries cannot come back. / **词库/历史复活**：单设备删除或清空会通过墓碑传播，远端旧条目无法复活。
+- **Flow false-ready mic state**: the keyboard mic now stays orange until the host app publishes a real ready contract (capture engine live + polling idle), not merely a fresh heartbeat; green tap-to-talk and jump-to-host behavior share the same `MicVoiceAvailability` gate, and orphaned `stopped` signals self-heal instead of hanging until timeout. / **Flow 伪就绪麦克风状态**：键盘麦克风在主 App 发布真实就绪合约（音频引擎在跑且轮询空闲）之前保持橙色，不再仅凭心跳误判；绿色「点按说话」与跳转主 App 共用同一 `MicVoiceAvailability` 闸门，孤立的 `stopped` 信号会自愈而不再长时间卡住。
+- **Flow mic stuck orange after ready**: a single stale cross-process heartbeat read no longer flips a healthy session into a sticky "session ended" error that forced the mic orange. The "session ended" hint now fires only when the (heartbeat-independent) session contract truly drops; a brief read jitter is smoothed by a ready grace window, and a lingering expired hint auto-recovers to green once the host is ready again. / **就绪后麦克风卡橙色**：单次跨进程心跳读数抖动不再把健康会话打成粘滞的「会话已结束」错误、强制麦克风变橙。「会话已结束」提示现仅在（不依赖心跳的）会话合约真正失效时触发；短暂读数抖动由就绪宽限期平滑，遗留的过期提示会在宿主重新就绪后自动恢复为绿色。
+
 ## [0.5.0] - 2026-07-07
 
 ### Added