feat: IAX2-inspired protocol improvements — trunking, mini-frames, silence suppression, call control (P2-T6/T7/T8/T9)

WZP-P2-T6: Trunking
- TrunkFrame/TrunkEntry: pack N session packets into one datagram
- Wire format: [count:u16][session_id:2][len:u16][payload]...
- TrunkBatcher: batches by count (10) or bytes (1200), flushes on limit
- 5 tests: encode/decode roundtrip, empty frame, batcher fill/flush, byte limit

WZP-P2-T7: Mini-frames
- MiniHeader: 4-byte delta header (timestamp_delta + payload_len)
- FRAME_TYPE_FULL (0x00) / FRAME_TYPE_MINI (0x01) discriminator
- MiniFrameContext: expands mini-headers to full by tracking baseline
- Saves 8 bytes per packet (5 vs 13 bytes with type prefix)
- 5 tests: encode/decode, wire size, context expand, no baseline, size comparison

WZP-P2-T8: Silence suppression
- SilenceDetector: RMS-based detection with hangover (5 frames = 100ms)
- ComfortNoise: low-level random noise generator
- CodecId::ComfortNoise variant for CN packets
- CallEncoder: suppresses silent frames, sends 1-byte CN every 200ms
- CallDecoder: generates comfort noise on CN packets
- ~50% bandwidth savings in typical conversations
- 6 tests: silence/speech detection, hangover, CN generation, RMS math, suppression

WZP-P2-T9: Call control signals
- SignalMessage: Hold, Unhold, Mute, Unmute, Transfer, TransferAck
- CallSignalType mapping in featherchat.rs for all new variants
- 4 serde roundtrip tests + signal type mapping tests

255 tests passing across all crates.

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

crates/wzp-client/src/featherchat.rs

@@ -26,6 +26,11 @@ pub enum CallSignalType {
     Reject,
     Ringing,
     Busy,
+    Hold,
+    Unhold,
+    Mute,
+    Unmute,
+    Transfer,
 }
 
 /// A CallSignal as sent through featherChat's WireMessage.
@@ -96,6 +101,12 @@ pub fn signal_to_call_type(signal: &SignalMessage) -> CallSignalType {
         SignalMessage::QualityUpdate { .. } => CallSignalType::Offer, // reuse
         SignalMessage::Ping { .. } | SignalMessage::Pong { .. } => CallSignalType::Offer,
         SignalMessage::AuthToken { .. } => CallSignalType::Offer,
+        SignalMessage::Hold => CallSignalType::Hold,
+        SignalMessage::Unhold => CallSignalType::Unhold,
+        SignalMessage::Mute => CallSignalType::Mute,
+        SignalMessage::Unmute => CallSignalType::Unmute,
+        SignalMessage::Transfer { .. } => CallSignalType::Transfer,
+        SignalMessage::TransferAck => CallSignalType::Offer, // reuse
     }
 }
 
@@ -134,5 +145,16 @@ mod tests {
             reason: wzp_proto::HangupReason::Normal,
         };
         assert!(matches!(signal_to_call_type(&hangup), CallSignalType::Hangup));
+
+        assert!(matches!(signal_to_call_type(&SignalMessage::Hold), CallSignalType::Hold));
+        assert!(matches!(signal_to_call_type(&SignalMessage::Unhold), CallSignalType::Unhold));
+        assert!(matches!(signal_to_call_type(&SignalMessage::Mute), CallSignalType::Mute));
+        assert!(matches!(signal_to_call_type(&SignalMessage::Unmute), CallSignalType::Unmute));
+
+        let transfer = SignalMessage::Transfer {
+            target_fingerprint: "abc".to_string(),
+            relay_addr: None,
+        };
+        assert!(matches!(signal_to_call_type(&transfer), CallSignalType::Transfer));
     }
 }