feat: complete WZP Phase 2 (T2/T3/T4) — adaptive quality, AudioWorklet, sessions

WZP-P2-T2: Adaptive quality switching
- QualityAdapter with sliding window of QualityReports
- Hysteresis: 3 consecutive reports before switching profiles
- Thresholds: loss>15%/rtt>200ms→CATASTROPHIC, loss>5%/rtt>100ms→DEGRADED
- CallConfig::from_profile() constructor
- 5 unit tests: good/degraded/catastrophic conditions, hysteresis, recovery

WZP-P2-T3: AudioWorklet migration (web bridge)
- audio-processor.js: WZPCaptureProcessor + WZPPlaybackProcessor
- Capture: buffers 128-sample AudioWorklet blocks → 960-sample frames
- Playback: ring buffer, Int16→Float32 conversion in worklet
- ScriptProcessorNode fallback if AudioWorklet unavailable
- Existing UI preserved (connect, room, PTT)

WZP-P2-T4: Concurrent session management (relay)
- SessionManager tracks active sessions with HashMap
- Enforces max_sessions limit from RelayConfig
- create_session/remove_session lifecycle
- Wired into relay main: session created after auth+handshake,
  cleaned up after run_participant returns
- 7 unit tests: create/remove, max enforced, room tracking, info, expiry

207 tests passing across all crates.

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

crates/wzp-relay/src/lib.rs

@@ -17,4 +17,4 @@ pub mod session_mgr;
 pub use config::RelayConfig;
 pub use handshake::accept_handshake;
 pub use pipeline::{PipelineConfig, PipelineStats, RelayPipeline};
-pub use session_mgr::{RelaySession, SessionId, SessionManager};
+pub use session_mgr::{RelaySession, SessionId, SessionInfo, SessionManager, SessionState};