[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/model_lifecycle.dart

@@ -94,6 +94,44 @@ class ModelLifecycle {
     return p.join(dir.path, config.filename);
   }
 
+  /// Lightweight ready estimate for warm-up gating (#311).
+  ///
+  /// Skips the SHA-256 re-hash that [checkAvailability] performs — for a
+  /// ~2.4GB model file the hash is wall-clock-noticeable on every screen
+  /// mount. Returns `ready` iff:
+  ///   - opt_in is true
+  ///   - download_state is not in-progress
+  ///   - meta_kv has both ai_model_path and ai_model_sha256
+  ///   - the file exists on disk
+  ///
+  /// Tampering/disk-corruption detection is left to [checkAvailability]'s
+  /// cold path (SettingsScreen). The trade-off is documented in
+  /// `docs/design/311-llm-warmup/README.md` §11 R4.
+  Future<ModelAvailability> quickCheck() async {
+    try {
+      final optIn = await meta.find(AiMetaKeys.optIn);
+      if (optIn != 'true') return ModelAvailability.missing;
+
+      final state = await meta.find(AiMetaKeys.downloadState);
+      if (state == 'downloading' || state == 'paused') {
+        return ModelAvailability.downloading;
+      }
+
+      final pathStr = await meta.find(AiMetaKeys.modelPath);
+      if (pathStr == null) return ModelAvailability.missing;
+
+      final expected = await meta.find(AiMetaKeys.modelSha);
+      if (expected == null) return ModelAvailability.corrupt;
+
+      final file = File(pathStr);
+      if (!file.existsSync()) return ModelAvailability.missing;
+
+      return ModelAvailability.ready;
+    } catch (_) {
+      return ModelAvailability.corrupt;
+    }
+  }
+
   Future<ModelAvailability> checkAvailability() async {
     try {
       final optIn = await meta.find(AiMetaKeys.optIn);