[03-Developer] #311 LLM warm-up + concurrent guard + quickCheck

ChatScreen 마운트 시 백그라운드 native init 으로 첫 send 시점에 native
load 지연을 안 보이게 한다. 12개 AC + UX-Reviewer 의 6개 권고 모두 코드
반영.

핵심 변경:
- `chat_warmup_provider.dart` — `ChatWarmupController` (Idle/Loading/Ready
  /Unavailable/Failed sealed state). fast path (`llm.isLoaded` → Ready),
  FileSystemException ↔ runtime kind 분기, _disposed race guard.
- `model_lifecycle.dart` — `quickCheck()`: 2.4GB SHA-256 hashing 없이
  meta_kv + 파일 존재만 보고 ready 추정 (R4 UX 권고).
- `gemma_llm_service.dart` + `llm_service.dart` — `_loadingFuture` 동시
  호출 가드. 두 caller 가 동시에 load() 해도 native init 은 1 회만.
- `chat_screen.dart` — initState postFrameCallback 에서 warmup.start().
  warmup 상태에 따라 hintText / spinner / 실패 banner 분기.

AC coverage (12개):
- AC1~AC8: ChatWarmupController unit (chat_warmup_test.dart 8 tests).
- AC9~AC12: UX-Reviewer 의 4개 권고 (입력 enabled / send auto-activate /
  fast path no-flicker / 명령형 메시지 금지) — controller 레벨에서 검증.

테스트: 167 passed (1 pre-existing skip). `flutter analyze` clean.

Refs #311

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/lib/data/ai/llm_service.dart

@@ -73,6 +73,16 @@ class MockLlmService implements LlmService {
   Map<String, dynamic>? lastSchema;
   Duration responseDelay = Duration.zero;
 
+  /// #311 test helpers. Simulate cold-load latency / failure so the warm-up
+  /// controller can be exercised. Mirrors the Gemma path:
+  /// - `loadDelay > 0`  → load completes after the delay
+  /// - `loadThrows`     → load throws this error
+  /// - `loadCount`      → observed by concurrent-load tests
+  Duration loadDelay = Duration.zero;
+  Object? loadThrows;
+  int loadCount = 0;
+  Future<void>? _loadingFuture;
+
   /// Queues consumed by [startChat] in order. Each entry is the event list
   /// returned for a single `send*` call.
   final List<List<LlmChatEvent>> chatScript = [];
@@ -82,8 +92,29 @@ class MockLlmService implements LlmService {
   @override
   bool get isLoaded => _loaded;
 
+  /// #311 AC7: same concurrent-call guard as [GemmaLlmService]. Repeated
+  /// in-flight `load()` calls share a single Future, so test assertions on
+  /// `loadCount` reflect the number of native-init attempts (1), not the
+  /// number of callers.
   @override
-  Future<void> load() async {
+  Future<void> load() {
+    if (_loaded) return Future.value();
+    final existing = _loadingFuture;
+    if (existing != null) return existing;
+    final future = _doLoad();
+    _loadingFuture = future;
+    return future.whenComplete(() {
+      _loadingFuture = null;
+    });
+  }
+
+  Future<void> _doLoad() async {
+    loadCount += 1;
+    if (loadDelay > Duration.zero) {
+      await Future<void>.delayed(loadDelay);
+    }
+    final err = loadThrows;
+    if (err != null) throw err;
     _loaded = true;
   }