feat(container): gate WebRTC on a decision resolved before the product realm

[filvecchiato]

Restores enforcement for RemotePermission::WebRtc, which #399 left advertised but unenforced. ICE reaches an arbitrary host over UDP, so no rule list, shouldInterceptRequest, or CSP directive sees it — this is the one egress path with no out-of-realm enforcement point.

• The host peeks the decision and bakes it in as a literal; the container removes RTCPeerConnection unless it is an explicit true. An async request from inside the product realm is forgeable (WebRTC gating #379), and removing the constructor leaves no reachable prototype to escape from. A fresh grant applies on the next load.
• Hosts must now inject the container into every frame (forMainFrameOnly: false). A subframe had no container at all, so innerHTML/document.write/srcdoc reached a realm with pristine fetch/WebSocket/RTCPeerConnection. The bootstrap stays main-frame-only, so subframes fail closed.
• 18 tests cover all four WebRTC gating #379 escapes, plus two that prove those routes do defeat a subclass gate so the suite cannot pass vacuously.