[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/state/chat_warmup_provider.dart

@@ -0,0 +1,135 @@
+import 'dart:io';
+
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../data/ai/llm_service.dart';
+import '../data/ai/model_lifecycle.dart';
+import 'ai_providers.dart';
+
+/// State machine for ChatScreen LLM warm-up (#311).
+///
+/// See `docs/design/311-llm-warmup/README.md` §6 / fn-chat_warmup_controller.md.
+sealed class ChatWarmupState {
+  const ChatWarmupState();
+}
+
+final class ChatWarmupIdle extends ChatWarmupState {
+  const ChatWarmupIdle();
+}
+
+final class ChatWarmupLoading extends ChatWarmupState {
+  const ChatWarmupLoading();
+}
+
+final class ChatWarmupReady extends ChatWarmupState {
+  const ChatWarmupReady();
+}
+
+/// Warm-up was not attempted because [ModelLifecycle.quickCheck] returned
+/// something other than `ready` (download incomplete, opt-out, corrupt).
+/// UI behaves as if warm-up didn't exist; the first user send falls back to
+/// the existing lazy `userTurn` path.
+final class ChatWarmupUnavailable extends ChatWarmupState {
+  const ChatWarmupUnavailable();
+}
+
+/// `kind` discriminates the retry copy: `fileMissing` is a settings-level
+/// recovery; `runtime` is a transient retry.
+enum ChatWarmupFailureKind { fileMissing, runtime }
+
+final class ChatWarmupFailed extends ChatWarmupState {
+  final String message;
+  final ChatWarmupFailureKind kind;
+  const ChatWarmupFailed(this.message, this.kind);
+}
+
+/// Drives `LlmService.load()` on ChatScreen mount so the first user send
+/// doesn't pay native-init latency. AC1-AC12 (12개) 모두 본 controller 가
+/// 흡수한다 (UI binding 은 chat_screen.dart 가 본 state 를 watch).
+class ChatWarmupController extends StateNotifier<ChatWarmupState> {
+  ChatWarmupController({
+    required this.llm,
+    required this.lifecycle,
+  }) : super(const ChatWarmupIdle());
+
+  final LlmService llm;
+  final ModelLifecycle lifecycle;
+  bool _disposed = false;
+
+  /// Idempotent. Re-entrant guard via the Loading state — duplicate `start`
+  /// calls during an in-flight load do nothing (the running future will set
+  /// the final state). External callers use [retry] instead.
+  Future<void> start() async {
+    if (state is ChatWarmupLoading) return;
+
+    // AC11 / UX R4: fast path. Skip Loading entirely if the underlying
+    // service is already loaded — prevents 1-frame label flicker on
+    // ChatScreen re-entry.
+    if (llm.isLoaded) {
+      _safeSet(const ChatWarmupReady());
+      return;
+    }
+
+    final availability = await lifecycle.quickCheck();
+    if (_disposed) return;
+    if (availability != ModelAvailability.ready) {
+      _safeSet(const ChatWarmupUnavailable());
+      return;
+    }
+
+    _safeSet(const ChatWarmupLoading());
+    try {
+      await llm.load();
+    } catch (e) {
+      if (_disposed) return;
+      final kind = e is FileSystemException
+          ? ChatWarmupFailureKind.fileMissing
+          : ChatWarmupFailureKind.runtime;
+      _safeSet(ChatWarmupFailed(_messageFor(kind), kind));
+      return;
+    }
+    if (_disposed) return;
+    _safeSet(const ChatWarmupReady());
+  }
+
+  Future<void> retry() async {
+    if (_disposed) return;
+    _safeSet(const ChatWarmupIdle());
+    await start();
+  }
+
+  /// AC6 / AC12: state는 disposed 인스턴스에는 더 이상 쓰지 않는다.
+  /// StateNotifier 의 setter 는 disposed 시 throw 하므로 가드 필수.
+  void _safeSet(ChatWarmupState s) {
+    if (_disposed) return;
+    state = s;
+  }
+
+  /// UX R5 / AC12: 메시지는 **상태**만 기술. "다시 시도해주세요" 같은
+  /// 명령형은 [다시 시도] 버튼이 담당하므로 본 문안에 넣지 않는다.
+  String _messageFor(ChatWarmupFailureKind kind) {
+    switch (kind) {
+      case ChatWarmupFailureKind.fileMissing:
+        return 'AI 모델 파일을 찾을 수 없어요.';
+      case ChatWarmupFailureKind.runtime:
+        return 'AI 를 시작하지 못했어요.';
+    }
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    super.dispose();
+  }
+}
+
+/// autoDispose: ChatScreen 이 pop 되면 controller 도 dispose → mount race 안전.
+final chatWarmupProvider =
+    StateNotifierProvider.autoDispose<ChatWarmupController, ChatWarmupState>(
+  (ref) {
+    return ChatWarmupController(
+      llm: ref.watch(llmServiceProvider),
+      lifecycle: ref.watch(modelLifecycleProvider),
+    );
+  },
+);