tauri-runtime-servocat 0.5.0

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. v0.5.0 adds an IPC bridge so JS scripts can call host commands via `__TAURI__.invoke(cmd, ...args)`, and back-propagates scripted DOM mutations into layout via `web_api_cat::extract_document`. tauri_runtime::Runtime trait impl is the remaining committed deliverable. The Servo no-AI policy disqualifies upstream contribution; this is the AI-built parallel.