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/SpeechHistoryEntry.swift

@@ -0,0 +1,32 @@
+// SpeechHistoryEntry.swift
+// OSGKeyboard · Shared
+//
+// A single voice transcription in the cross-device history log.
+
+import Foundation
+
+public struct SpeechHistoryEntry: Codable, Identifiable, Equatable, Sendable {
+    public let id: UUID
+    public let text: String
+    public let createdAt: Date
+    /// iOS Flow engine mode; nil on macOS captures.
+    public let engineMode: String?
+
+    public init(
+        id: UUID = UUID(),
+        text: String,
+        createdAt: Date = Date(),
+        engineMode: String? = nil
+    ) {
+        self.id = id
+        self.text = text
+        self.createdAt = createdAt
+        self.engineMode = engineMode
+    }
+
+    /// First-line preview for compact list rows (macOS history sidebar).
+    public var previewTitle: String {
+        let firstLine = text.split(separator: "\n").first.map(String.init) ?? text
+        return firstLine.count > 36 ? String(firstLine.prefix(36)) + "…" : firstLine
+    }
+}