Add bidirectional host bridge

docs/protocol.md

@@ -92,6 +92,22 @@ Network results are delivered back to Lua through `on_event(event)`:
 - `network_ws_error`: WebSocket connection or stream error.
 - `network_ws_close`: WebSocket connection closed.
 
+## Runtime host bridge
+
+Flutter host apps may register a `RuntimeHostBridge` when creating `LuaGameWidget` or `FlameLuaGame`.
+
+Lua-to-Flutter calls:
+
+- `runtime.host_call({ id?, method, data? })`: async request. Result is delivered to Lua as `host_call_result` with `id`, `method`, `ok`, and either `result` or `error`.
+- `runtime.host_notify({ method, data? })`: fire-and-forget notification to Flutter host code.
+
+Flutter-to-Lua calls:
+
+- `FlameLuaGame.notifyLua(method, data?)`: emits a `host_notify` event into Lua.
+- `FlameLuaGame.callLua(method, data?, timeout?)`: emits a `host_call` event into Lua and waits for Lua to call `runtime.host_respond({ id, result?, error? })`.
+
+Host bridge payloads must be JSON-like values: null, bool, number, string, list, or string-keyed map. Unsupported Dart objects are converted to strings.
+
 ## RuntimeCommand
 
 Runtime commands request generic side effects owned by Dart/Flame.