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/Models/SyncedField.swift

@@ -0,0 +1,29 @@
+// SyncedField.swift
+// OSGKeyboard · Shared
+//
+// Per-field metadata for conflict-free settings merge across devices.
+
+import Foundation
+
+public struct SyncedField<T: Codable & Equatable & Sendable>: Codable, Equatable, Sendable {
+    public var value: T
+    public var updatedAt: Date
+    public var deviceID: String
+
+    public init(value: T, updatedAt: Date = Date(), deviceID: String) {
+        self.value = value
+        self.updatedAt = updatedAt
+        self.deviceID = deviceID
+    }
+
+    /// Pick the field with the newer `updatedAt`; ties break lexicographically on `deviceID`.
+    public static func merge(local: SyncedField<T>, remote: SyncedField<T>) -> SyncedField<T> {
+        if remote.updatedAt > local.updatedAt { return remote }
+        if local.updatedAt > remote.updatedAt { return local }
+        return remote.deviceID >= local.deviceID ? remote : local
+    }
+
+    public static func make(value: T, deviceID: String) -> SyncedField<T> {
+        SyncedField(value: value, updatedAt: Date(), deviceID: deviceID)
+    }
+}