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)

crates/chanora_bridge/src/api.rs

@@ -1486,6 +1486,15 @@ pub async fn download_avatar(
         .map_err(BridgeError::from)
 }
 
+/// Resolve icon bytes through the bridge.
+pub async fn download_icon(icon_id: u64) -> Result<Option<Vec<u8>>, BridgeError> {
+    runtime()
+        .spawn(async move { session().get_icon(icon_id).await })
+        .await
+        .map_err(|e| task_join_error("download_icon", e))?
+        .map_err(BridgeError::from)
+}
+
 /// Purge cached protocol-owned assets.
 pub async fn clear_file_cache() -> Result<(), BridgeError> {
     runtime()