Implements the core runtime-created window contract from #67. Parent/modal semantics remain separate follow-up decisions. The full CI matrix is green on tip 1279f74.
- Add named-window creation/open from the native CLI and page bridge on Linux and Windows, with stable handles, addressed controls and reads, and close/reopen cleanup.
- Authenticate each sending WebKitGTK page or WebView2 event subscription, route correlated replies back to its owner page, and emit scoped lifecycle events.
- Exercise the shipped Linux binary under Xvfb and the Windows binary with WebView2 through create, concurrent child opens, resize, close, stale-handle rejection, and same-name reopen.
- Ship a checksum-pinned WebView2 loader in Windows release archives and scan it with native artifacts.
- Update platform and lifecycle documentation for the verified behavior.
Verification: Linux and Windows GUI smokes passed on the latest code commit; the current tip changes documentation only. TypeScript SDK CI, typecheck, package verification, focused tests, and pickier are green. The full CI matrix passed on 1279f74, including native Linux and Windows GUI smokes and the mobile E2E jobs. Local full SDK tests have unrelated Bun-canary relocated-bundle timeout and HMR failures; the SDK CI job passes.