[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/data/ai/model_lifecycle.dart

@@ -233,21 +233,37 @@ class ModelLifecycle {
   /// opt-out: delete model file + clear all ai_* meta keys (except opt_in
   /// which the caller toggles). Returns freed bytes (0 if nothing existed).
   /// Idempotent.
+  ///
+  /// F2 hardening (#218): per-file try/catch so a single OS-level delete
+  /// failure (locked file, permission flake) does not abort the whole
+  /// purge — meta keys still get cleared and the orphan file becomes a
+  /// background storage concern rather than a stuck "opt-out failed"
+  /// state. The freed-bytes count only reflects successful deletes.
   Future<int> purge() async {
     int freed = 0;
     final pathStr = await meta.find(AiMetaKeys.modelPath);
     if (pathStr != null) {
-      final f = File(pathStr);
-      if (f.existsSync()) {
-        freed += await f.length();
-        await f.delete();
+      try {
+        final f = File(pathStr);
+        if (f.existsSync()) {
+          final size = await f.length();
+          await f.delete();
+          freed += size;
+        }
+      } catch (_) {
+        // Best-effort; leave orphan file, continue purging meta.
       }
     }
-    final tempPath = '${await _modelPath()}.tmp';
-    final temp = File(tempPath);
-    if (temp.existsSync()) {
-      freed += await temp.length();
-      await temp.delete();
+    try {
+      final tempPath = '${await _modelPath()}.tmp';
+      final temp = File(tempPath);
+      if (temp.existsSync()) {
+        final size = await temp.length();
+        await temp.delete();
+        freed += size;
+      }
+    } catch (_) {
+      // Same as above — best-effort cleanup of the .tmp partial.
     }
     for (final k in [
       AiMetaKeys.modelPath,
@@ -255,7 +271,12 @@ class ModelLifecycle {
       AiMetaKeys.downloadState,
       AiMetaKeys.downloadBytes,
     ]) {
-      await meta.remove(k);
+      try {
+        await meta.remove(k);
+      } catch (_) {
+        // Meta is a single sqlite table; failures here are rare.
+        // Swallow so the loop completes even if one key errors.
+      }
     }
     return freed;
   }