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/ICloudSync/SyncDeviceID.swift

@@ -0,0 +1,20 @@
+// SyncDeviceID.swift
+// OSGKeyboard · Shared
+//
+// Stable per-install identifier for per-field / per-device iCloud merge.
+
+import Foundation
+
+public enum SyncDeviceID {
+    private static let defaultsKey = "sync.deviceID.v1"
+
+    /// Returns a stable device id stored in the active defaults suite.
+    public static func current(defaults: UserDefaults = AppGroupStore().defaults) -> String {
+        if let existing = defaults.string(forKey: defaultsKey), !existing.isEmpty {
+            return existing
+        }
+        let created = UUID().uuidString
+        defaults.set(created, forKey: defaultsKey)
+        return created
+    }
+}