Support Firefox via engine-aware channel routing

[vc0d35]

Firefox's ICE stack refuses to dial loopback candidates, so loopback ports

= 1024 fall back to fetch (which on Firefox resolves for any open port, not
just HTTP); LAN and remote high ports still use ICE as on Chromium. The open oracle now keys on candidate-pair state (in-progress/succeeded), which both engines populate, instead of the Chromium-only requestsSent.

• batch.js: isChromium() + isLoopbackHost() gate the ICE/fetch split
• probeWithIce.js: unified state-based oracle
• tests: mock emits state; new loopback-fallback case; ICE-routing tests use a non-loopback host; e2e runs Firefox with loopback-ICE specs gated to Chromium; CI installs Firefox
• README: three-way per-engine routing, oracle, and caveats