[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/ui/screens/chat_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../ai/tools/tool_envelope.dart';
 import '../../state/chat_providers.dart';
+import '../../state/chat_warmup_provider.dart';
 
 /// AI chat surface (#260). Multi-turn tool calling powered by Gemma 4 +
 /// in-process tool runtime. ConfirmGate modals appear on destructive
@@ -18,6 +19,18 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
   final _textCtrl = TextEditingController();
   final _scrollCtrl = ScrollController();
 
+  @override
+  void initState() {
+    super.initState();
+    // #311 AC1: ChatScreen mount → background warm-up. depsAsync.data 가
+    // resolve 되기 전에는 toolDepsProvider 도 미준비라 send 자체가 막혀
+    // 있으므로, 그 사이에 native init 만 먼저 끝낸다.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted) return;
+      ref.read(chatWarmupProvider.notifier).start();
+    });
+  }
+
   @override
   void dispose() {
     _textCtrl.dispose();
@@ -72,18 +85,33 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
 
   Widget _buildBody(BuildContext context) {
     final state = ref.watch(chatSessionControllerProvider);
+    final warmup = ref.watch(chatWarmupProvider);
     _scrollToBottom();
+
+    // #311 AC3 / UX R3: warmup 중에는 hintText 만 교체. 입력창 자체는
+    // enabled (사용자가 미리 타이핑 가능 — AC9).
+    final isWarming = warmup is ChatWarmupLoading;
+    final hintText = isWarming
+        ? 'AI 준비 중… 첫 시작은 몇 초 걸려요'
+        : '습관 추가, 기록, 카탈로그 질문…';
+
+    // AC10: warmup ready 이고 streaming 중이 아닐 때 send 활성. 빈 텍스트는
+    // _send() 가 early-return 하므로 별도 gating 불필요 (rebuild race 회피).
+    final canSend = !state.isStreaming && !isWarming;
+    final theme = Theme.of(context);
+
     return Column(
       children: [
+        if (warmup is ChatWarmupFailed) _WarmupErrorBanner(warmup: warmup),
         if (state.error != null)
           Container(
             width: double.infinity,
-            color: Theme.of(context).colorScheme.errorContainer,
+            color: theme.colorScheme.errorContainer,
             padding: const EdgeInsets.all(12),
             child: Text(
               state.error!,
               style: TextStyle(
-                color: Theme.of(context).colorScheme.onErrorContainer,
+                color: theme.colorScheme.onErrorContainer,
               ),
             ),
           ),
@@ -117,31 +145,32 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
                 child: TextField(
                   controller: _textCtrl,
                   enabled: !state.isStreaming,
-                  decoration: const InputDecoration(
-                    hintText: '습관 추가, 기록, 카탈로그 질문…',
-                    border: OutlineInputBorder(),
+                  decoration: InputDecoration(
+                    hintText: hintText,
+                    border: const OutlineInputBorder(),
                     isDense: true,
                   ),
                   maxLines: 4,
                   minLines: 1,
                   textInputAction: TextInputAction.send,
-                  onSubmitted: (_) => _send(),
+                  onSubmitted: (_) => canSend ? _send() : null,
                 ),
               ),
               const SizedBox(width: 8),
-              state.isStreaming
-                  ? const Padding(
-                      padding: EdgeInsets.all(8),
-                      child: SizedBox(
-                        width: 24,
-                        height: 24,
-                        child: CircularProgressIndicator(strokeWidth: 2),
-                      ),
-                    )
-                  : IconButton.filled(
-                      onPressed: _send,
-                      icon: const Icon(Icons.send),
-                    ),
+              if (state.isStreaming || isWarming)
+                const Padding(
+                  padding: EdgeInsets.all(8),
+                  child: SizedBox(
+                    width: 24,
+                    height: 24,
+                    child: CircularProgressIndicator(strokeWidth: 2),
+                  ),
+                )
+              else
+                IconButton.filled(
+                  onPressed: canSend ? _send : null,
+                  icon: const Icon(Icons.send),
+                ),
             ],
           ),
         ),
@@ -150,6 +179,41 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
   }
 }
 
+/// #311 AC5 / UX R5+R6: 실패 메시지는 상태만 기술, 행동은 [다시 시도] 버튼.
+class _WarmupErrorBanner extends ConsumerWidget {
+  final ChatWarmupFailed warmup;
+  const _WarmupErrorBanner({required this.warmup});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final theme = Theme.of(context);
+    return Container(
+      width: double.infinity,
+      color: theme.colorScheme.errorContainer,
+      padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 10),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          Text(
+            warmup.message,
+            style: TextStyle(color: theme.colorScheme.onErrorContainer),
+          ),
+          const SizedBox(height: 8),
+          Align(
+            alignment: Alignment.centerRight,
+            child: OutlinedButton(
+              onPressed: () {
+                ref.read(chatWarmupProvider.notifier).retry();
+              },
+              child: const Text('다시 시도'),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
 /// Human-friendly Korean labels for the 6 tools registered in
 /// `ToolRegistry.defaults()`. Falls back to the raw tool name for any
 /// future tool that hasn't been mapped yet — better to show the raw id