WebKit engine binding for vibesurfer.

This crate hosts the [Engine] trait — the daemon's view of "a browser" — and the [EngineRuntime] that pins the implementation to a dedicated OS thread (the only thread that may call WebKit on either supported platform).

Backends

• [backend::webkit] (macOS) — system WebKit framework via objc2 + objc2-web-kit, driven from the Cocoa main thread.
• [backend::wpe] (Linux) — WebKitGTK 6 via webkit6 + glib, driven from a thread that owns a GLib main context.
• [backend::webview2] (Windows) — WebView2 via webview2-com + windows-rs, driven from a thread that owns a Win32 message pump.

All backends implement [Engine]; [EngineRuntime::spawn] is generic over the constructor so the daemon can pick a backend at runtime without compile-time coupling.