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-proto/src/codec_id.rs

@@ -16,6 +16,8 @@ pub enum CodecId {
     Codec2_3200 = 3,
     /// Codec2 at 1200bps (catastrophic conditions)
     Codec2_1200 = 4,
+    /// Comfort noise descriptor (silence suppression)
+    ComfortNoise = 5,
 }
 
 impl CodecId {
@@ -27,6 +29,7 @@ impl CodecId {
             Self::Opus6k => 6_000,
             Self::Codec2_3200 => 3_200,
             Self::Codec2_1200 => 1_200,
+            Self::ComfortNoise => 0,
         }
     }
 
@@ -38,6 +41,7 @@ impl CodecId {
             Self::Opus6k => 40,
             Self::Codec2_3200 => 20,
             Self::Codec2_1200 => 40,
+            Self::ComfortNoise => 20,
         }
     }
 
@@ -46,6 +50,7 @@ impl CodecId {
         match self {
             Self::Opus24k | Self::Opus16k | Self::Opus6k => 48_000,
             Self::Codec2_3200 | Self::Codec2_1200 => 8_000,
+            Self::ComfortNoise => 48_000,
         }
     }
 
@@ -57,6 +62,7 @@ impl CodecId {
             2 => Some(Self::Opus6k),
             3 => Some(Self::Codec2_3200),
             4 => Some(Self::Codec2_1200),
+            5 => Some(Self::ComfortNoise),
             _ => None,
         }
     }