Diagnostic for Phase A. Chrome will not accept a hand-crafted SDP via setLocalDescription, so this page uses pc.createOffer() to genuinely reach have-local-offer state, then calls setRemoteDescription with our captured answer bytes. Two error shapes possible: "Invalid SDP line" (same as real failure -- SDP parse bug reproduced) vs ICE-ufrag/fingerprint mismatch (SDP parsed OK, real bug is elsewhere).
Edit to test variants. ICE ufrag/pwd/fingerprint will NOT match the local PC's generated offer; that mismatch is expected and is a separate signal from a parser failure.