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/MicVoiceAvailability.swift

@@ -0,0 +1,37 @@
+// MicVoiceAvailability.swift
+// OSGKeyboard · Shared
+//
+// Single source of truth for keyboard mic color, hint text, and tap behavior.
+
+import Foundation
+
+/// Whether the keyboard mic can start a Flow utterance right now.
+public enum MicVoiceAvailability: Equatable, Sendable {
+    /// Green — tap records immediately without opening the host app.
+    case ready
+    /// Orange — voice input blocked; see `Reason` for hint copy.
+    case unavailable(Reason)
+    /// Red — user is actively recording.
+    case recording
+    /// White — waiting for ASR / cloud polish after stop.
+    case processing
+
+    public enum Reason: Equatable, Sendable {
+        case missingAPIKey
+        case hostNotReady
+        case noFullAccess
+        case appGroupUnavailable
+        /// User tapped mic; host app jump in progress, awaiting ready contract.
+        case preparingSession
+    }
+
+    public var isReady: Bool {
+        if case .ready = self { return true }
+        return false
+    }
+
+    public var isUnavailable: Bool {
+        if case .unavailable = self { return true }
+        return false
+    }
+}