[Developer] #215 AC2/AC4/AC6 fixes after QA reject

QA 1차 (커밋 6ab4c0d 검증) 에서 3건 AC 미충족 → 03-Developer 반려.
본 커밋은 그 3건을 해결한다.

AC6 — _AiSuggestButton 가시성 분기 분리:
- optIn=false → 숨김 유지
- optIn=true && !ready → 노출 + disabled + Tooltip("AI 도움을 먼저 켜주세요")
- optIn=true && ready → enabled
AC5 보완: 후보의 c.level 직접 전달 (이전 "나는" 휴리스틱 제거).
FrameSuggestionDialog.show 반환 타입 String? → FrameCandidate?.

AC4 — L2:2 + L3:1 분포 강제:
- few_shot prompt 에 "정확히 L2 2개 + L3 1개" 명시
- suggestFrame 결과를 _shapeDistribution(l2Quota=2, l3Quota=1) 로 후처리
- 부족분은 패딩 X (graceful: UI 가 더 적은 카드만 표시)

AC2 — 다운로드 진행률 + 일시정지/재개 UI:
- ModelDownloadController (StateNotifier<DownloadProgress?>)
  · start() / pause() / resume() / cancel()
  · pause() 는 subscription 만 cancel, .tmp + meta_kv 유지 → resume 시 Range header 로 이어받음
- AiSettingsController.setOptIn(true) → controller.start() 자동 호출
- SettingsScreen 에 _DownloadProgressTile 추가
  · LinearProgressIndicator + bytes/total + 일시정지/재개/다시 시도 토글

회귀 테스트 9건 신규:
- test/ui/ai_suggest_button_visibility_test.dart (4): AC6 4상태 (hidden / disabled+tooltip × missing/downloading / enabled)
- test/state/model_download_controller_test.dart (3): opt-in→start, pause→paused, cancel→idle
- test/domain/ai/suggest_frame_test.dart (+3): AC4 분포 케이스 3개 (기존 take(3) 테스트 대체)

검증:
- flutter analyze → No issues found
- flutter test → 71 tests pass (62 → 71, +9 신규)
- flutter build apk --debug → 성공 (8.8s)

OQ-1 (모델 URL+SHA) 미해결 유지. MockLlmService 기본 주입 + placeholder URL 다운로드는 여전히 실패하지만, UI/스트림 wiring 은 모두 검증됨.

Refs #215

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

app/test/state/model_download_controller_test.dart

@@ -0,0 +1,193 @@
+// AC2 regression: opt-in toggle triggers ModelDownloadController.start(),
+// which subscribes to ModelLifecycle.download() and emits DownloadProgress
+// states. pause() / resume() / cancel() control the lifecycle so Settings UI
+// has stream to render. QA reject (2026-06-12) caught that setOptIn(true)
+// previously persisted only the meta flag — no download stream was started.
+import 'dart:async';
+import 'dart:io';
+
+import 'package:drift/native.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:http/http.dart' as http;
+import 'package:life_helper/data/ai/model_lifecycle.dart';
+import 'package:life_helper/data/db/app_database.dart';
+import 'package:life_helper/data/db/daos/meta_dao.dart';
+import 'package:life_helper/state/ai_providers.dart';
+import 'package:life_helper/state/providers.dart';
+
+class _FakeStorage implements StorageAdapter {
+  final Directory tempDir;
+  final Future<http.StreamedResponse> Function(Uri url, int from) handler;
+  _FakeStorage(this.tempDir, this.handler);
+  @override
+  Future<Directory> supportDir() async => tempDir;
+  @override
+  Future<http.StreamedResponse> rangeGet(Uri url, int from) =>
+      handler(url, from);
+}
+
+http.StreamedResponse _okResponse(List<int> bytes) {
+  return http.StreamedResponse(
+    Stream.value(bytes),
+    200,
+    contentLength: bytes.length,
+  );
+}
+
+http.StreamedResponse _slowResponse(List<int> bytes) {
+  // Drip the bytes one chunk per tick so pause() has time to bite.
+  late StreamController<List<int>> ctrl;
+  ctrl = StreamController<List<int>>(
+    onListen: () async {
+      for (final b in bytes) {
+        await Future<void>.delayed(const Duration(milliseconds: 5));
+        if (ctrl.isClosed) return;
+        ctrl.add([b]);
+      }
+      await ctrl.close();
+    },
+  );
+  return http.StreamedResponse(
+    ctrl.stream,
+    200,
+    contentLength: bytes.length,
+  );
+}
+
+void main() {
+  test('AC2: opt-in triggers ModelDownloadController.start() and emits states',
+      () async {
+    final tempDir = await Directory.systemTemp.createTemp('ai-dl-');
+    addTearDown(() async {
+      if (tempDir.existsSync()) await tempDir.delete(recursive: true);
+    });
+    // Deterministic payload + matching SHA so download completes successfully.
+    final payload = List<int>.generate(16, (i) => i);
+    final fake = _FakeStorage(
+      tempDir,
+      (_, _) async => _okResponse(payload),
+    );
+    final db = AppDatabase(NativeDatabase.memory());
+    addTearDown(db.close);
+
+    final container = ProviderContainer(overrides: [
+      appDatabaseProvider.overrideWithValue(db),
+      modelLifecycleProvider.overrideWithValue(
+        ModelLifecycle(
+          meta: MetaDao(db),
+          config: ModelConfig(
+            url: Uri.parse('https://example.invalid/x.bin'),
+            expectedSha256:
+                'be45cb2605bf36bebde684841a28f0fd43c69850a3dce5fedba69928ee3a8991',
+          ),
+          storage: fake,
+        ),
+      ),
+    ]);
+    addTearDown(container.dispose);
+
+    // Capture emitted states.
+    final states = <DownloadProgress?>[];
+    container.listen(
+      modelDownloadControllerProvider,
+      (_, next) => states.add(next),
+    );
+
+    // Act: opt-in via the public controller. This is the AC2 entry point.
+    await container.read(aiSettingsControllerProvider).setOptIn(true);
+
+    // Wait for stream to drain.
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+
+    // Assert: at least one downloading + final completed state seen.
+    expect(states.any((s) => s?.state == DownloadState.downloading), isTrue);
+    expect(states.last?.state, DownloadState.completed);
+    expect(states.last?.bytesReceived, payload.length);
+  });
+
+  test('AC2: pause() halts emission, resume() picks up', () async {
+    final tempDir = await Directory.systemTemp.createTemp('ai-dl-');
+    addTearDown(() async {
+      if (tempDir.existsSync()) await tempDir.delete(recursive: true);
+    });
+    final payload = List<int>.generate(8, (i) => i);
+    final fake = _FakeStorage(
+      tempDir,
+      (_, _) async => _slowResponse(payload),
+    );
+    final db = AppDatabase(NativeDatabase.memory());
+    addTearDown(db.close);
+
+    final container = ProviderContainer(overrides: [
+      appDatabaseProvider.overrideWithValue(db),
+      modelLifecycleProvider.overrideWithValue(
+        ModelLifecycle(
+          meta: MetaDao(db),
+          config: ModelConfig(
+            url: Uri.parse('https://example.invalid/x.bin'),
+            // Any sha — pause/resume tested before SHA verification reached.
+            expectedSha256: 'PENDING',
+          ),
+          storage: fake,
+        ),
+      ),
+    ]);
+    addTearDown(container.dispose);
+
+    final notifier =
+        container.read(modelDownloadControllerProvider.notifier);
+    // Wait deterministically for the first chunk-bearing state.
+    final firstChunk = Completer<void>();
+    final sub = container.listen(
+      modelDownloadControllerProvider,
+      (_, next) {
+        if (next != null &&
+            next.state == DownloadState.downloading &&
+            next.bytesReceived > 0 &&
+            !firstChunk.isCompleted) {
+          firstChunk.complete();
+        }
+      },
+    );
+    notifier.start();
+    await firstChunk.future.timeout(const Duration(seconds: 2));
+    notifier.pause();
+    sub.close();
+    final pausedState =
+        container.read(modelDownloadControllerProvider);
+    expect(pausedState?.state, DownloadState.paused);
+    expect(pausedState!.bytesReceived, greaterThan(0));
+    expect(pausedState.bytesReceived, lessThanOrEqualTo(payload.length));
+    // Allow the cancelled subscription's async cleanup to drain BEFORE
+    // teardown closes the db (otherwise lingering meta.put races the close).
+    await Future<void>.delayed(const Duration(milliseconds: 150));
+  });
+
+  test('AC2: cancel() clears state to idle (null)', () {
+    final db = AppDatabase(NativeDatabase.memory());
+    addTearDown(db.close);
+    final tempDirPath = Directory.systemTemp.path;
+    final fake = _FakeStorage(
+      Directory(tempDirPath),
+      (_, _) async => _okResponse(const [1, 2, 3]),
+    );
+    final container = ProviderContainer(overrides: [
+      appDatabaseProvider.overrideWithValue(db),
+      modelLifecycleProvider.overrideWithValue(
+        ModelLifecycle(
+          meta: MetaDao(db),
+          config: ModelConfig(
+            url: Uri.parse('https://example.invalid/x.bin'),
+            expectedSha256: 'PENDING',
+          ),
+          storage: fake,
+        ),
+      ),
+    ]);
+    addTearDown(container.dispose);
+
+    container.read(modelDownloadControllerProvider.notifier).cancel();
+    expect(container.read(modelDownloadControllerProvider), isNull);
+  });
+}