feat(dred): continuous DRED tuning, PMTUD, extended Opus6k window

- DredTuner: maps live network metrics (loss/RTT/jitter) to continuous
  DRED duration every ~500ms instead of discrete tier-locked values.
  Includes jitter-spike detection for pre-emptive Starlink-style boost.
- Opus6k DRED extended from 500ms to 1040ms (max libopus 1.5 supports)
- PMTUD: quinn MtuDiscoveryConfig with upper_bound=1452, 300s interval
- TrunkedForwarder respects discovered MTU (was hard-coded 1200)
- QuinnPathSnapshot exposes quinn internal stats + discovered MTU
- AudioEncoder trait: set_expected_loss() + set_dred_duration() methods
- PathMonitor: sliding-window jitter variance for spike detection
- Integrated into both Android and desktop send tasks in engine.rs
- 14 new tests (10 tuner unit + 4 encoder integration)
- Updated ARCHITECTURE.md, PROGRESS.md, PRD-dred-integration, PRD-mtu

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/tests/cross_relay_direct_call.rs

@@ -52,6 +52,7 @@ fn alice_offer(call_id: &str) -> SignalMessage {
         supported_profiles: vec![],
         caller_reflexive_addr: Some(ALICE_ADDR.into()),
         caller_local_addrs: Vec::new(),
+        caller_build_version: None,
     }
 }
 
@@ -132,6 +133,7 @@ fn bob_answer(call_id: &str) -> SignalMessage {
         chosen_profile: None,
         callee_reflexive_addr: Some(BOB_ADDR.into()),
         callee_local_addrs: Vec::new(),
+        callee_build_version: None,
     }
 }