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>

docs/PRD-mtu-discovery.md

@@ -57,3 +57,28 @@ When the path MTU is small, the relay or client should:
 - MTU-based codec selection (future, needs adaptive quality)
 
 ## Effort: 1 day
+
+## Implementation Status (2026-04-12)
+
+Phase 1 is now implemented:
+
+### What was built
+
+- **Transport config** (`crates/wzp-transport/src/config.rs`):
+  - `MtuDiscoveryConfig` with `upper_bound=1452`, `interval=300s`, `black_hole_cooldown=30s`
+  - `initial_mtu=1200` (safe QUIC minimum)
+  - Quinn's PLPMTUD binary-searches from 1200 up to 1452 automatically
+
+- **`QuinnPathSnapshot::current_mtu`** (`crates/wzp-transport/src/quic.rs`):
+  - Reads `connection.max_datagram_size()` which reflects the PMTUD-discovered value
+  - Available to all callers via `transport.quinn_path_stats()`
+
+- **Trunk batcher MTU-aware** (`crates/wzp-relay/src/room.rs`):
+  - `TrunkedForwarder::new()` initializes `max_bytes` from discovered MTU
+  - `send()` refreshes `max_bytes` on every call (cheap atomic read in quinn)
+  - Federation trunk frames grow automatically as PMTUD discovers larger paths
+
+### Phases 2-3 status
+
+- Phase 2 (handle MTU failures): Already handled — `send_media()`/`send_trunk()` check `max_datagram_size()` and return `DatagramTooLarge` errors. These are logged and the packet is dropped gracefully.
+- Phase 3 (codec-aware MTU): Not yet implemented. Future video frames will need application-layer fragmentation when they exceed the discovered MTU.