tauri-runtime-servocat 1.5.1

Servo-replacement runtime for Tauri: wires html-cat, css-cat, dom-cat, layout-cat, paint-cat, net-cat, boa-cat, ecma-runtime-cat, and web-api-cat into a single rendering + scripting pipeline. v1.5.1 wires the JS side of the IPC bridge: when a webview's script calls `__TAURI__.post_ipc_message(payload)`, our `NativeFn` shim fires the same `WebviewIpcHandler` that `send_ipc(request)` does, so a Tauri app's `tauri::Builder::invoke_handler` is reachable from JS running in boa-cat. Thread-local proxy state threads through the bare-fn-pointer `NativeFn` constraint; otherwise no shared mutable state. The Servo no-AI policy disqualifies upstream contribution; this is the AI-built parallel.