feat(flutter): add cache initialization wiring and avatar download shims

- Add wireCache() to app_bootstrap using getApplicationCacheDirectory()
- Call wireCache() after wireStorage() in main bootstrap flow
- Add Dart-side initCache and downloadAvatar wrapper shims in api.dart
- Update Cargo.lock for new chanora_cache dependency

apps/chanora_flutter/lib/services/app_bootstrap.dart

@@ -15,15 +15,22 @@ typedef StorageDirectoryProvider = Future<Directory> Function();
 typedef StorageInitializer = Future<void> Function(String dir);
 
 Future<void>? _storageInitFuture;
+Future<void>? _cacheInitFuture;
 Future<void>? _vadBootstrapFuture;
 StorageDirectoryProvider _storageDirectoryProvider =
     getApplicationSupportDirectory;
+StorageDirectoryProvider _cacheDirectoryProvider = getApplicationCacheDirectory;
 StorageInitializer _storageInitializer = _defaultStorageInitializer;
+StorageInitializer _cacheInitializer = _defaultCacheInitializer;
 
 Future<void> _defaultStorageInitializer(String dir) {
   return rust.initStorage(dir: dir);
 }
 
+Future<void> _defaultCacheInitializer(String dir) {
+  return rust.initCache(dir: dir);
+}
+
 Future<File> _copyBundledAssetToDocuments({
   required String assetPath,
   required String fileName,
@@ -121,16 +128,50 @@ Future<void> _wireStorageImpl() async {
   }
 }
 
+Future<void> wireCache() async {
+  final existing = _cacheInitFuture;
+  if (existing != null) {
+    await existing;
+    return;
+  }
+
+  final initFuture = _wireCacheImpl();
+  _cacheInitFuture = initFuture;
+  await initFuture;
+}
+
+Future<void> _wireCacheImpl() async {
+  var initialized = false;
+  try {
+    final dir = await _cacheDirectoryProvider();
+    await _cacheInitializer(dir.path);
+    initialized = true;
+  } catch (_) {
+    // Best-effort; missing cache just means protocol-owned assets are
+    // re-downloaded this session.
+  } finally {
+    if (!initialized) {
+      _cacheInitFuture = null;
+    }
+  }
+}
+
 @visibleForTesting
 void debugResetStorageBootstrap({
   StorageDirectoryProvider? storageDirectoryProvider,
+  StorageDirectoryProvider? cacheDirectoryProvider,
   StorageInitializer? storageInitializer,
+  StorageInitializer? cacheInitializer,
 }) {
   _storageInitFuture = null;
+  _cacheInitFuture = null;
   _vadBootstrapFuture = null;
   _storageDirectoryProvider =
       storageDirectoryProvider ?? getApplicationSupportDirectory;
+  _cacheDirectoryProvider =
+      cacheDirectoryProvider ?? getApplicationCacheDirectory;
   _storageInitializer = storageInitializer ?? _defaultStorageInitializer;
+  _cacheInitializer = cacheInitializer ?? _defaultCacheInitializer;
 }
 
 rust.BridgeNetworkState _mapConnectivity(List<ConnectivityResult> results) {