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/sweep.rs

@@ -89,6 +89,7 @@ pub fn run_local_sweep(config: &SweepConfig) -> Vec<SweepResult> {
                 jitter_target: target,
                 jitter_max: max,
                 jitter_min: target.min(3).max(1),
+                ..Default::default()
             };
 
             let mut encoder = CallEncoder::new(&call_cfg);