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.

OSGKeyboardTests/ConfigurationStoreTests.swift

@@ -0,0 +1,44 @@
+// ConfigurationStoreTests.swift
+// OSGKeyboardTests
+//
+// Locks `AppGroupStore` conformance to `ConfigurationStore` and ensures
+// pipeline helpers accept the protocol without changing iOS behavior.
+
+import XCTest
+@testable import OSGKeyboardShared
+
+final class ConfigurationStoreTests: XCTestCase {
+    private var suiteName: String!
+    private var defaults: UserDefaults!
+    private var store: AppGroupStore!
+
+    override func setUp() {
+        super.setUp()
+        suiteName = "group.com.osgkeyboard.shared.tests.configuration.\(UUID().uuidString)"
+        defaults = UserDefaults(suiteName: suiteName)!
+        defaults.removePersistentDomain(forName: suiteName)
+        store = AppGroupStore(defaults: defaults)
+    }
+
+    override func tearDown() {
+        defaults.removePersistentDomain(forName: suiteName)
+        defaults = nil
+        store = nil
+        super.tearDown()
+    }
+
+    func testAppGroupStoreConformsToConfigurationStore() {
+        let configuration: any ConfigurationStore = store
+        XCTAssertEqual(configuration.cloudASRPersistence, defaults)
+        XCTAssertEqual(
+            PolishingService.resolvedProviderId(store: configuration, providerIdOverride: nil),
+            PolishingService.resolvedProviderId(store: store, providerIdOverride: nil)
+        )
+    }
+
+    func testASRFactoryAcceptsConfigurationStore() {
+        store.setEngineMode("local")
+        let service = ASRServiceFactory.make(store: store as any ConfigurationStore)
+        XCTAssertTrue(service is SpeechAnalyzerASR)
+    }
+}