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. v3.15.0 closes the async arc end-to-end: bumps to boa-cat 0.6 (Promise type + microtask driver + `await` evaluator) + ecma-runtime-cat 0.3 (`Promise.{resolve,reject,all,race}` global) + web-api-cat 0.5, and proves the wiring with an end-to-end test that has a `HostCommands` callback return a `Value::Promise` and a JS script `await invoke('cmd', ...)` to unwrap it. The existing `NativeFn` host-command surface is unchanged -- promises returned from commands now compose naturally with `.then` / `await` / `Promise.all` through the engine's dispatch path.