[Developer] #218 Real Gemma 4 E2B integration via flutter_gemma 0.16.5

Implements the OQ-1 follow-up to #215 v0.2.0: replace the placeholder
GemmaLlmService stub with a real flutter_gemma 0.16.5 backend driving
Gemma 4 E2B (litert-community/gemma-4-E2B-it-litert-lm, 2.41GB).

Highlights:
- GemmaLlmService.load → FlutterGemma.initialize + installModel.fromFile +
  getActiveModel; idempotent + FileSystemException on missing file.
- generateStructured uses Gemma 4 native function calling via
  createChat(tools: [Tool(...)], toolChoice: required). Stream parsed by
  collectFunctionCall — first FCR wins, ParallelFCR first-call wins,
  TextResponse/ThinkingResponse skipped, errors sanitized to prevent
  prompt leakage.
- main.dart wires _LazyLlmService adapter that resolves to GemmaLlmService
  when ModelLifecycle reports ready, MockLlmService otherwise.
- ai_providers.dart pins real model URL + SHA-256 (181938...39a63c).
- F2 hardening: ModelLifecycle.purge wraps each delete + meta remove in
  try/catch so a single OS-level flake cannot block opt-out.
- Android: INTERNET / FOREGROUND_SERVICE / POST_NOTIFICATIONS permissions
  + R8 proguard-rules.pro keeping MediaPipe / LiteRT / TFLite / protobuf
  JNI entry points (release builds otherwise crash on first inference).

Design-First: fn-gemma_llm_service.md updated to v2 — §C
(_appendSchemaInstruction) deprecated after reading flutter_gemma
0.16.5 source (Gemma 4 SDK injects tool declarations via template;
prompt-side append would double-wrap).

Tests:
- 10 new unit tests for collectFunctionCall covering all 8 fn-spec
  cases + 2 ParallelFunctionCallResponse paths.
- All 81 existing tests still pass.
- flutter analyze: 0 issues.

Refs #218

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

app/lib/state/ai_providers.dart

@@ -10,19 +10,24 @@ import '../domain/ai/suggest_frame.dart';
 import '../domain/models/frame_pattern.dart';
 import 'providers.dart';
 
-/// Default config for the on-device Gemma model (#215).
-/// OQ-1: URL + SHA-256 pinned in Developer phase. Until then, downloads are
-/// disabled (AI toggle is gated behind these constants being real).
-const _kModelUrlPlaceholder =
-    'https://example.invalid/gemma4-e2b-q4.bin'; // OQ-1
-const _kModelShaPlaceholder = 'PENDING_OQ_1';
+/// Gemma 4 E2B instruction-tuned LiteRT-LM checkpoint (#218 OQ-1 resolved).
+/// Hosted on HuggingFace `litert-community/gemma-4-E2B-it-litert-lm`.
+/// File ≈ 2.41GB; SHA-256 pinned for integrity check.
+///
+/// Tests / placeholder builds may override `modelLifecycleProvider` with
+/// fixture URLs. Production builds optionally inject a private mirror via
+/// `--dart-define=GEMMA_MODEL_URL=...` (see main.dart).
+const _kModelUrl =
+    'https://huggingface.co/litert-community/gemma-4-E2B-it-litert-lm/resolve/main/gemma-4-E2B-it.litertlm';
+const _kModelSha256 =
+    '181938105e0eefd105961417e8da75903eacda102c4fce9ce90f50b97139a63c';
 
 final modelLifecycleProvider = Provider<ModelLifecycle>((ref) {
   return ModelLifecycle(
     meta: ref.watch(metaDaoProvider),
     config: ModelConfig(
-      url: Uri.parse(_kModelUrlPlaceholder),
-      expectedSha256: _kModelShaPlaceholder,
+      url: Uri.parse(_kModelUrl),
+      expectedSha256: _kModelSha256,
     ),
   );
 });