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-client/src/featherchat.rs

@@ -134,6 +134,10 @@ pub fn signal_to_call_type(signal: &SignalMessage) -> CallSignalType {
         SignalMessage::CandidateUpdate { .. } => CallSignalType::IceCandidate, // mid-call re-gather
         SignalMessage::HardNatProbe { .. } => CallSignalType::IceCandidate, // hard NAT coordination
         SignalMessage::HardNatBirthdayStart { .. } => CallSignalType::IceCandidate, // birthday attack
+        SignalMessage::UpgradeProposal { .. }
+        | SignalMessage::UpgradeResponse { .. }
+        | SignalMessage::UpgradeConfirm { .. }
+        | SignalMessage::QualityCapability { .. } => CallSignalType::Offer, // quality negotiation
         SignalMessage::QualityDirective { .. } => CallSignalType::Offer, // relay-initiated
     }
 }