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/MacQwen3LocalASR.swift

@@ -0,0 +1,39 @@
+// MacQwen3LocalASR.swift
+// OSGKeyboard · Mac
+//
+// Qwen3-ASR-1.7B (MLX) via mlx-swift-asr. Expects a converted model directory
+// containing config.json, model.safetensors, and tokenizer files.
+
+import Foundation
+
+enum MacQwen3LocalASR {
+    /// Transcribe with Qwen3-ASR MLX weights at `modelPath`.
+    static func transcribe(
+        samples: [Float],
+        sampleRate: Int,
+        locale: Locale,
+        modelPath: String
+    ) async throws -> String {
+        guard MacLocalASRPreferences.qwen3ModelIsInstalled(at: modelPath) else {
+            throw MacLocalASRError.qwen3ModelMissing
+        }
+        guard sampleRate == 16_000 else {
+            throw MacLocalASRError.qwen3InferenceFailed(
+                "Qwen3-ASR expects 16 kHz audio (got \(sampleRate) Hz)"
+            )
+        }
+
+        let language = MacQwen3LanguageHint.from(locale: locale)
+        do {
+            return try await MacQwen3ASREngine.shared.transcribe(
+                samples: samples,
+                language: language,
+                modelPath: modelPath
+            )
+        } catch let error as MacLocalASRError {
+            throw error
+        } catch {
+            throw MacLocalASRError.qwen3InferenceFailed(error.localizedDescription)
+        }
+    }
+}