[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/lib/data/ai/model_lifecycle.dart

@@ -66,11 +66,16 @@ class _ProdStorage implements StorageAdapter {
 class ModelConfig {
   final Uri url;
   final String expectedSha256;
+
+  /// MUST keep the `.litertlm` extension: LiteRT-LM sniffs the container
+  /// format from the file extension, and a `.bin` name makes native
+  /// engine_create fail with "Model may be invalid" on every backend
+  /// (#342 root cause, verified on-emulator 2026-07-09).
   final String filename;
   const ModelConfig({
     required this.url,
     required this.expectedSha256,
-    this.filename = 'gemma4_e2b_q4.bin',
+    this.filename = 'gemma-4-E2B-it.litertlm',
   });
 }
 
@@ -94,6 +99,26 @@ class ModelLifecycle {
     return p.join(dir.path, config.filename);
   }
 
+  /// #657 AC-2/3/4: pre-.litertlm installs saved the model as
+  /// `gemma4_e2b_q4.bin`, which LiteRT-LM's engine_create rejects. Renaming
+  /// the existing file (same bytes — SHA meta stays valid) spares those
+  /// users a 2.4GB re-download. Idempotent; a failed rename leaves state
+  /// untouched so the next availability check retries.
+  Future<void> _migrateLegacyPath() async {
+    final metaPath = await meta.find(AiMetaKeys.modelPath);
+    if (metaPath == null) return;
+    final canonical = await _modelPath();
+    if (metaPath == canonical) return;
+    final legacy = File(metaPath);
+    if (!legacy.existsSync()) return;
+    try {
+      await legacy.rename(canonical);
+    } catch (_) {
+      return;
+    }
+    await meta.put(AiMetaKeys.modelPath, canonical);
+  }
+
   /// Lightweight ready estimate for warm-up gating (#311).
   ///
   /// Skips the SHA-256 re-hash that [checkAvailability] performs — for a
@@ -117,6 +142,8 @@ class ModelLifecycle {
         return ModelAvailability.downloading;
       }
 
+      await _migrateLegacyPath();
+
       final pathStr = await meta.find(AiMetaKeys.modelPath);
       if (pathStr == null) return ModelAvailability.missing;
 
@@ -142,6 +169,8 @@ class ModelLifecycle {
         return ModelAvailability.downloading;
       }
 
+      await _migrateLegacyPath();
+
       final pathStr = await meta.find(AiMetaKeys.modelPath);
       if (pathStr == null) return ModelAvailability.missing;