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.

OSGKeyboardMac/MacICloudSyncBootstrap.swift

@@ -0,0 +1,40 @@
+// MacICloudSyncBootstrap.swift
+// OSGKeyboard · Mac
+//
+// Wires the shared iCloud KVS sync layer to macOS UserDefaults so settings
+// and personal dictionary stay aligned with the iOS app.
+
+import Foundation
+
+@MainActor
+enum MacICloudSyncBootstrap {
+    private static var configured = false
+    private static var cloudSync: AppCloudSync?
+
+    static func configure(defaults: UserDefaults) {
+        guard !configured else { return }
+        configured = true
+        let makeStore = { AppGroupStore(defaults: defaults) }
+        cloudSync = AppCloudSync(makeStore: makeStore, historyDefaults: { defaults })
+        cloudSync?.startObservingExternalChanges()
+    }
+
+    static func pullIfEnabled() async {
+        await cloudSync?.pullAllIfEnabled()
+    }
+
+    static var settingsSync: SettingsCloudSync {
+        if let cloudSync {
+            return cloudSync.settingsSyncService
+        }
+        return SettingsCloudSync(makeStore: { AppGroupStore(defaults: .standard) })
+    }
+
+    static var dictionarySync: PersonalDictionaryCloudSync {
+        cloudSync?.dictionarySyncService ?? PersonalDictionaryCloudSync(makeStore: { AppGroupStore(defaults: .standard) })
+    }
+
+    static var appCloudSync: AppCloudSync {
+        cloudSync ?? AppCloudSync.shared
+    }
+}