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/session_mgr.rs

@@ -1,6 +1,7 @@
 //! Session manager — tracks active call sessions on the relay.
 
 use std::collections::HashMap;
+use std::time::Instant;
 
 use wzp_proto::{QualityProfile, Session};
 
@@ -9,6 +10,26 @@ use crate::pipeline::{PipelineConfig, RelayPipeline};
 /// Unique identifier for a relay session.
 pub type SessionId = [u8; 16];
 
+/// Lifecycle state of a concurrent session.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SessionState {
+    Active,
+    Closing,
+}
+
+/// Lightweight metadata for a concurrent session (room-mode tracking).
+#[derive(Debug, Clone)]
+pub struct SessionInfo {
+    /// Which room this session belongs to.
+    pub room_name: String,
+    /// Client fingerprint (present when auth is enabled).
+    pub fingerprint: Option<String>,
+    /// When the session was created.
+    pub connected_at: Instant,
+    /// Current lifecycle state.
+    pub state: SessionState,
+}
+
 /// A single active call session on the relay.
 pub struct RelaySession {
     /// Protocol session state machine.
@@ -47,8 +68,14 @@ impl RelaySession {
 }
 
 /// Manages all active sessions on a relay.
+///
+/// Combines two layers of tracking:
+/// - `sessions`: heavy `RelaySession` objects (pipeline state machines, used in forward mode)
+/// - `tracked`: lightweight `SessionInfo` entries (room + fingerprint, used in room mode to
+///   enforce `max_sessions` and answer lifecycle queries)
 pub struct SessionManager {
     sessions: HashMap<SessionId, RelaySession>,
+    tracked: HashMap<SessionId, SessionInfo>,
     max_sessions: usize,
 }
 
@@ -56,17 +83,20 @@ impl SessionManager {
     pub fn new(max_sessions: usize) -> Self {
         Self {
             sessions: HashMap::new(),
+            tracked: HashMap::new(),
             max_sessions,
         }
     }
 
-    /// Create a new session. Returns None if at capacity.
-    pub fn create_session(
+    // ── Heavy session API (forward-mode pipelines) ──────────────────────
+
+    /// Create a new pipeline session. Returns None if at capacity.
+    pub fn create_pipeline_session(
         &mut self,
         session_id: SessionId,
         config: PipelineConfig,
     ) -> Option<&mut RelaySession> {
-        if self.sessions.len() >= self.max_sessions {
+        if self.total_count() >= self.max_sessions {
             return None;
         }
         self.sessions
@@ -75,53 +105,124 @@ impl SessionManager {
         self.sessions.get_mut(&session_id)
     }
 
-    /// Get a session by ID.
+    /// Get a pipeline session by ID.
     pub fn get_session(&mut self, id: &SessionId) -> Option<&mut RelaySession> {
         self.sessions.get_mut(id)
     }
 
-    /// Remove a session.
-    pub fn remove_session(&mut self, id: &SessionId) -> Option<RelaySession> {
+    /// Remove a pipeline session.
+    pub fn remove_pipeline_session(&mut self, id: &SessionId) -> Option<RelaySession> {
         self.sessions.remove(id)
     }
 
-    /// Number of active sessions.
-    pub fn active_count(&self) -> usize {
+    /// Number of active pipeline sessions.
+    pub fn pipeline_active_count(&self) -> usize {
         self.sessions.values().filter(|s| s.is_active()).count()
     }
 
-    /// Total sessions (including inactive/closing).
-    pub fn total_count(&self) -> usize {
+    /// Total pipeline sessions (including inactive/closing).
+    pub fn pipeline_total_count(&self) -> usize {
         self.sessions.len()
     }
 
-    /// Remove sessions idle for longer than `timeout_ms`.
+    /// Remove pipeline sessions idle for longer than `timeout_ms`.
     pub fn expire_idle(&mut self, now_ms: u64, timeout_ms: u64) -> usize {
         let before = self.sessions.len();
         self.sessions
             .retain(|_, s| now_ms.saturating_sub(s.last_activity_ms) < timeout_ms);
         before - self.sessions.len()
     }
+
+    // ── Lightweight concurrent-session API (room mode) ──────────────────
+
+    /// Register a new concurrent session.
+    /// Returns the `SessionId` on success, or an error string if `max_sessions` is exceeded.
+    pub fn create_session(
+        &mut self,
+        room: &str,
+        fingerprint: Option<String>,
+    ) -> Result<SessionId, String> {
+        if self.total_count() >= self.max_sessions {
+            return Err(format!(
+                "max sessions ({}) exceeded",
+                self.max_sessions
+            ));
+        }
+        let id = rand_session_id();
+        self.tracked.insert(id, SessionInfo {
+            room_name: room.to_string(),
+            fingerprint,
+            connected_at: Instant::now(),
+            state: SessionState::Active,
+        });
+        Ok(id)
+    }
+
+    /// Remove a tracked session.
+    pub fn remove_session(&mut self, id: SessionId) {
+        self.tracked.remove(&id);
+    }
+
+    /// Number of currently tracked (room-mode) sessions.
+    pub fn active_count(&self) -> usize {
+        self.tracked.values().filter(|s| s.state == SessionState::Active).count()
+    }
+
+    /// Return all session IDs that belong to a given room.
+    pub fn sessions_in_room(&self, room: &str) -> Vec<SessionId> {
+        self.tracked
+            .iter()
+            .filter(|(_, info)| info.room_name == room)
+            .map(|(id, _)| *id)
+            .collect()
+    }
+
+    /// Get metadata for a tracked session.
+    pub fn session_info(&self, id: SessionId) -> Option<&SessionInfo> {
+        self.tracked.get(&id)
+    }
+
+    /// Total sessions across both tracking layers.
+    pub fn total_count(&self) -> usize {
+        self.sessions.len() + self.tracked.len()
+    }
+}
+
+/// Generate a random 16-byte session identifier.
+fn rand_session_id() -> SessionId {
+    let mut id = [0u8; 16];
+    // Use a simple monotonic + random source to avoid pulling in `rand` crate.
+    // Hash the instant + a counter for uniqueness.
+    use std::sync::atomic::{AtomicU64, Ordering};
+    static CTR: AtomicU64 = AtomicU64::new(1);
+    let ctr = CTR.fetch_add(1, Ordering::Relaxed);
+    let bytes = ctr.to_le_bytes();
+    id[..8].copy_from_slice(&bytes);
+    // Mix in some time-based entropy for the upper half.
+    let t = Instant::now().elapsed().as_nanos() as u64;
+    id[8..16].copy_from_slice(&t.to_le_bytes());
+    id
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    // ── Pipeline session tests (pre-existing, adapted to renamed API) ───
+
     #[test]
-    fn create_and_get_session() {
+    fn create_and_get_pipeline_session() {
         let mut mgr = SessionManager::new(10);
         let id = [1u8; 16];
-        mgr.create_session(id, PipelineConfig::default());
-        assert_eq!(mgr.total_count(), 1);
+        mgr.create_pipeline_session(id, PipelineConfig::default());
         assert!(mgr.get_session(&id).is_some());
     }
 
     #[test]
-    fn respects_max_sessions() {
+    fn respects_max_pipeline_sessions() {
         let mut mgr = SessionManager::new(1);
-        mgr.create_session([1u8; 16], PipelineConfig::default());
-        let result = mgr.create_session([2u8; 16], PipelineConfig::default());
+        mgr.create_pipeline_session([1u8; 16], PipelineConfig::default());
+        let result = mgr.create_pipeline_session([2u8; 16], PipelineConfig::default());
         assert!(result.is_none());
     }
 
@@ -129,10 +230,73 @@ mod tests {
     fn expire_idle_removes_old() {
         let mut mgr = SessionManager::new(10);
         let id = [1u8; 16];
-        mgr.create_session(id, PipelineConfig::default());
-        // Session has last_activity_ms = 0, current time = 60000, timeout = 30000
+        mgr.create_pipeline_session(id, PipelineConfig::default());
         let expired = mgr.expire_idle(60_000, 30_000);
         assert_eq!(expired, 1);
-        assert_eq!(mgr.total_count(), 0);
+        assert_eq!(mgr.pipeline_total_count(), 0);
+    }
+
+    // ── Concurrent session (room-mode) tests ────────────────────────────
+
+    #[test]
+    fn create_and_remove() {
+        let mut mgr = SessionManager::new(10);
+        let id = mgr.create_session("room-a", Some("fp123".into())).unwrap();
+        assert_eq!(mgr.active_count(), 1);
+        mgr.remove_session(id);
+        assert_eq!(mgr.active_count(), 0);
+    }
+
+    #[test]
+    fn max_sessions_enforced() {
+        let mut mgr = SessionManager::new(2);
+        mgr.create_session("r1", None).unwrap();
+        mgr.create_session("r2", None).unwrap();
+        let err = mgr.create_session("r3", None);
+        assert!(err.is_err());
+        assert!(err.unwrap_err().contains("max sessions"));
+    }
+
+    #[test]
+    fn sessions_in_room_tracking() {
+        let mut mgr = SessionManager::new(10);
+        let a1 = mgr.create_session("alpha", None).unwrap();
+        let _a2 = mgr.create_session("alpha", None).unwrap();
+        let _b1 = mgr.create_session("beta", None).unwrap();
+
+        let alpha_ids = mgr.sessions_in_room("alpha");
+        assert_eq!(alpha_ids.len(), 2);
+        assert!(alpha_ids.contains(&a1));
+
+        let beta_ids = mgr.sessions_in_room("beta");
+        assert_eq!(beta_ids.len(), 1);
+
+        let empty = mgr.sessions_in_room("gamma");
+        assert!(empty.is_empty());
+    }
+
+    #[test]
+    fn session_info_returns_correct_data() {
+        let mut mgr = SessionManager::new(10);
+        let id = mgr.create_session("room-x", Some("alice-fp".into())).unwrap();
+
+        let info = mgr.session_info(id).expect("session should exist");
+        assert_eq!(info.room_name, "room-x");
+        assert_eq!(info.fingerprint.as_deref(), Some("alice-fp"));
+        assert_eq!(info.state, SessionState::Active);
+
+        // Non-existent session returns None
+        assert!(mgr.session_info([0xFFu8; 16]).is_none());
+    }
+
+    #[test]
+    fn max_sessions_shared_across_both_layers() {
+        let mut mgr = SessionManager::new(2);
+        // One pipeline session + one tracked session = 2 = at capacity
+        mgr.create_pipeline_session([1u8; 16], PipelineConfig::default());
+        mgr.create_session("room", None).unwrap();
+        // Both layers should now reject
+        assert!(mgr.create_session("room", None).is_err());
+        assert!(mgr.create_pipeline_session([2u8; 16], PipelineConfig::default()).is_none());
     }
 }