New SignalMessage variants for P2P quality coordination:
UpgradeProposal/UpgradeResponse/UpgradeConfirm (#28):
- Consensual quality upgrade flow — proposer sends desired profile,
peer accepts/rejects based on own conditions, confirm commits both
- All carry call_id for relay routing
QualityCapability (#30):
- Peer reports its max sustainable profile — enables asymmetric
encoding where each side uses its own best quality instead of
forcing everyone to the weakest link
Relay forwards all 4 signals to the call peer (same pattern as
MediaPathReport, CandidateUpdate, HardNatProbe).
Desktop signal recv loop handles all 4 with debug logging.
Encoder switching TODOs noted for wiring into CallEngine.
4 new serde roundtrip tests. 603 total, 0 regressions.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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