[Developer] #657 모델 파일명 .litertlm fix + 레거시 마이그레이션 + 에뮬레이터 dev 인프라

- ModelConfig.filename 기본값 gemma4_e2b_q4.bin → gemma-4-E2B-it.litertlm (#342 근본 원인)
- _migrateLegacyPath: 기존 .bin 설치본 rename + meta 갱신 (재다운로드 2.4GB 방지, 멱등)
- maxTokens 2048 유지 + 1024 회귀 방지 주석 (KV cache < prefill signature 시 추론 실패)
- LLM_BACKEND dart-define: 에뮬레이터 CPU 강제 (SwiftShader GPU 네이티브 SIGABRT 회피)
- GEMMA_MODEL_URL dart-define: 호스트 로컬 모델 서버 주입 (기본값 = HF URL 불변)
- debug 전용 cleartext manifest (10.0.2.2 모델 서버용, release 불변)
- 마이그레이션 테스트 4건 신규 (AC-2/3/4). 171 passed, analyze clean

2026-07-14 에뮬레이터 E2E 검증: 다운로드→로드→tool call 왕복 전 구간 성공.

Refs #657, #342

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/test/data/ai/model_lifecycle_test.dart

@@ -250,4 +250,80 @@ void main() {
     await meta.put(AiMetaKeys.modelSha, 'expected_but_actual_will_differ');
     expect(await lc.checkAvailability(), ModelAvailability.corrupt);
   });
+
+  group('#657 legacy .bin filename migration', () {
+    // Pre-.litertlm installs: file on disk + meta path both use the old
+    // `gemma4_e2b_q4.bin` name that LiteRT-LM rejects.
+    const legacyName = 'gemma4_e2b_q4.bin';
+    const canonicalName = 'model.litertlm';
+
+    ModelLifecycle makeLc(String expectedSha) => ModelLifecycle(
+          meta: meta,
+          config: ModelConfig(
+            url: Uri.parse(url),
+            expectedSha256: expectedSha,
+            filename: canonicalName,
+          ),
+          storage: storage,
+        );
+
+    Future<String> seedLegacy(List<int> payload) async {
+      final legacyPath = '${tmp.path}/$legacyName';
+      File(legacyPath).writeAsBytesSync(payload);
+      await meta.put(AiMetaKeys.optIn, 'true');
+      await meta.put(AiMetaKeys.modelPath, legacyPath);
+      await meta.put(
+          AiMetaKeys.modelSha, sha256.convert(payload).toString());
+      await meta.put(AiMetaKeys.downloadState, 'completed');
+      return legacyPath;
+    }
+
+    test('quickCheck renames legacy file + updates meta path (AC-2)',
+        () async {
+      final payload = utf8.encode('legacy model bytes');
+      final legacyPath = await seedLegacy(payload);
+      final lc = makeLc(sha256.convert(payload).toString());
+
+      expect(await lc.quickCheck(), ModelAvailability.ready);
+      expect(File(legacyPath).existsSync(), isFalse);
+      final canonicalPath = '${tmp.path}/$canonicalName';
+      expect(File(canonicalPath).existsSync(), isTrue);
+      expect(await meta.find(AiMetaKeys.modelPath), canonicalPath);
+    });
+
+    test('checkAvailability migrates and SHA meta stays valid (AC-2)',
+        () async {
+      final payload = utf8.encode('legacy model bytes');
+      await seedLegacy(payload);
+      final lc = makeLc(sha256.convert(payload).toString());
+
+      expect(await lc.checkAvailability(), ModelAvailability.ready);
+      expect(File('${tmp.path}/$canonicalName').existsSync(), isTrue);
+    });
+
+    test('migration is idempotent — second check is a no-op (AC-3)',
+        () async {
+      final payload = utf8.encode('legacy model bytes');
+      await seedLegacy(payload);
+      final lc = makeLc(sha256.convert(payload).toString());
+
+      expect(await lc.quickCheck(), ModelAvailability.ready);
+      expect(await lc.quickCheck(), ModelAvailability.ready);
+      expect(
+        await meta.find(AiMetaKeys.modelPath),
+        '${tmp.path}/$canonicalName',
+      );
+    });
+
+    test('meta path file gone → skip migration, stays missing (AC-4)',
+        () async {
+      await meta.put(AiMetaKeys.optIn, 'true');
+      await meta.put(AiMetaKeys.modelPath, '${tmp.path}/$legacyName');
+      await meta.put(AiMetaKeys.modelSha, 'irrelevant');
+
+      final lc = makeLc('irrelevant');
+      expect(await lc.quickCheck(), ModelAvailability.missing);
+      expect(File('${tmp.path}/$canonicalName').existsSync(), isFalse);
+    });
+  });
 }