feat(quality): upgrade negotiation + asymmetric quality signals (#28, #29, #30)

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>

crates/wzp-relay/src/main.rs

@@ -1446,7 +1446,11 @@ async fn main() -> anyhow::Result<()> {
                                 // Hard NAT: forward HardNatProbe + HardNatBirthdayStart
                                 // to call peer (same pattern as CandidateUpdate).
                                 SignalMessage::HardNatBirthdayStart { ref call_id, .. } |
-                                SignalMessage::HardNatProbe { ref call_id, .. } => {
+                                SignalMessage::HardNatProbe { ref call_id, .. } |
+                                SignalMessage::UpgradeProposal { ref call_id, .. } |
+                                SignalMessage::UpgradeResponse { ref call_id, .. } |
+                                SignalMessage::UpgradeConfirm { ref call_id, .. } |
+                                SignalMessage::QualityCapability { ref call_id, .. } => {
                                     let (peer_fp, peer_relay_fp) = {
                                         let reg = call_registry.lock().await;
                                         match reg.get(call_id) {