feat: file transfer system (avatar/icon download with cacache) (#40)

* docs(architecture): add file transfer design, research, and implementation plan

* feat(cache): add chanora_cache crate with cacache-backed blob cache

- New chanora_cache crate: content-addressed blob store wrapping cacache
- BlobCache API: async put/get/remove/clear/total_size/evict
- Key validation: av_ prefix (32 hex chars), ic_ prefix (decimal digits)
- Cacache provides crash safety, SSRI integrity, content dedup
- Mtime-based eviction via cacache::list_sync + sort by timestamp
- 7 unit tests all passing
- Added to workspace members

* feat(protocol): add file download support for avatars and icons

- Add Request::DownloadFile variant with oneshot reply
- Add ProtocolClient::download_avatar(client_uid) and download_icon(icon_id)
- Track pending file downloads by FiletransferHandle
- Handle StreamItem::FileDownload: read bytes from TCP stream
- Handle StreamItem::FiletransferFailed: map to ProtocolError
- Add ProtocolError::FileTransfer(String) variant
- Add path helper tests for avatar/icon download paths
- No tsclientlib types leak across the adapter boundary

* feat(core): add blob cache wiring and avatar download orchestration

- Add chanora_cache dependency to Cargo.toml
- Add blob_cache field to ChanoraSession (Arc<Mutex<Option<BlobCache>>>)
- Add init_cache() method: creates BlobCache, runs eviction
- Add get_avatar() method: cache-first, download on miss, store in cache
- Add clear_cache() and cache_size() methods for cache management
- Add CoreError::Cache variant for BlobCacheError conversion
- Add avatar_cache integration test

* feat(bridge): add init_cache, download_avatar, and cache management functions

- Add init_cache(dir) bridge function
- Add download_avatar(avatar_hash, client_uid) bridge function
- Add clear_file_cache() and file_cache_size() bridge functions
- Map CoreError::Cache and ProtocolError::FileTransfer in BridgeError

* 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

* feat(core): FileTransferService with coalescing, throttling, negative cache

- New file_transfer module with FileTransferService struct
- Semaphore(2) throttles concurrent downloads
- In-flight HashMap coalesces duplicate avatar requests
- 5-min negative cache short-circuits ServerRejected misses
- ChanoraSession delegates get_avatar through the service
- connect/disconnect update shared protocol handle
- clear_cache/cache_size delegate to service
- 2 new unit tests (cached hit, negative cache)

* feat(core,bridge): add get_icon with coalescing and negative cache

- FileTransferService::get_icon() mirrors get_avatar pattern
- ChanoraSession::get_icon() delegates through FileTransferService
- Bridge download_icon() exposed for Flutter
- Dart downloadIcon() shim added
- Uses PREFIX_ICON (ic_<crc32u>) cache key format
- 1 new unit test (cached icon hit)

* fix(core,protocol): simplify store_protocol and add download size cap

- store_protocol: always write to shared Arc<Mutex<Option<ProtocolClient>>>;
  the FileTransferService holds the same Arc so it sees updates automatically
- read_download_bytes: reject downloads exceeding 10 MB to prevent
  malicious servers from causing OOM

Cargo.toml

@@ -10,6 +10,7 @@
 #   crates/chanora_resolver/     — TeamSpeak address resolution
 #   crates/chanora_state/        — snapshot, deltas, reducers
 #   crates/chanora_audio/        — capture, DSP, Opus, jitter, mixer
+#   crates/chanora_cache/        — avatar/icon blob cache (cacache-backed)
 #   crates/chanora_storage/      — bookmarks, settings, identity refs
 #   crates/chanora_diagnostics/  — logs, redaction, export
 #   crates/chanora_prefetch — server-resolution prefetch cache/policy
@@ -30,6 +31,7 @@ members = [
     "crates/chanora_state",
     "crates/chanora_audio",
     "crates/chanora_storage",
+    "crates/chanora_cache",
     "crates/chanora_diagnostics",
     "crates/chanora_prefetch",
     "crates/chanora_bridge",