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-codec/src/lib.rs

@@ -15,8 +15,10 @@ pub mod codec2_enc;
 pub mod opus_dec;
 pub mod opus_enc;
 pub mod resample;
+pub mod silence;
 
 pub use adaptive::{AdaptiveDecoder, AdaptiveEncoder};
+pub use silence::{ComfortNoise, SilenceDetector};
 pub use wzp_proto::{AudioDecoder, AudioEncoder, CodecId, QualityProfile};
 
 /// Create an adaptive encoder starting at the given quality profile.

crates/wzp-codec/src/silence.rs

@@ -0,0 +1,191 @@
+//! Silence suppression and comfort noise generation.
+//!
+//! During silent periods (~50% of a typical call), full encoded frames waste
+//! bandwidth. [`SilenceDetector`] detects silent audio based on RMS energy,
+//! and [`ComfortNoise`] generates low-level background noise to fill gaps on
+//! the decoder side.
+
+use rand::Rng;
+
+/// Detects silence in PCM audio using RMS energy with a hangover period.
+///
+/// The hangover prevents clipping the onset of speech: after silence is first
+/// detected, the detector continues reporting "not silent" for `hangover_frames`
+/// additional frames before transitioning to suppression.
+pub struct SilenceDetector {
+    /// RMS threshold below which audio is considered silent (for i16 samples).
+    threshold_rms: f64,
+    /// Number of frames to keep sending after silence starts (prevents speech clipping).
+    hangover_frames: u32,
+    /// Count of consecutive frames whose RMS is below the threshold.
+    silent_frames: u32,
+    /// Whether suppression is currently active.
+    is_suppressing: bool,
+}
+
+impl SilenceDetector {
+    /// Create a new silence detector.
+    ///
+    /// * `threshold_rms` — RMS energy below which a frame is silent (default: 100.0 for i16).
+    /// * `hangover_frames` — frames to keep sending after silence onset (default: 5 = 100ms at 20ms frames).
+    pub fn new(threshold_rms: f64, hangover_frames: u32) -> Self {
+        Self {
+            threshold_rms,
+            hangover_frames,
+            silent_frames: 0,
+            is_suppressing: false,
+        }
+    }
+
+    /// Compute the RMS (root mean square) energy of a PCM buffer.
+    pub fn rms(pcm: &[i16]) -> f64 {
+        if pcm.is_empty() {
+            return 0.0;
+        }
+        let sum_sq: f64 = pcm.iter().map(|&s| (s as f64) * (s as f64)).sum();
+        (sum_sq / pcm.len() as f64).sqrt()
+    }
+
+    /// Returns `true` if the frame should be suppressed (i.e. is silence past
+    /// the hangover period).
+    ///
+    /// Call once per frame. The detector tracks consecutive silent frames
+    /// internally and only reports suppression after the hangover expires.
+    pub fn is_silent(&mut self, pcm: &[i16]) -> bool {
+        let energy = Self::rms(pcm);
+
+        if energy < self.threshold_rms {
+            self.silent_frames = self.silent_frames.saturating_add(1);
+
+            if self.silent_frames > self.hangover_frames {
+                self.is_suppressing = true;
+            }
+        } else {
+            // Speech detected — reset.
+            self.silent_frames = 0;
+            self.is_suppressing = false;
+        }
+
+        self.is_suppressing
+    }
+
+    /// Whether the detector is currently in the suppressing state.
+    pub fn suppressing(&self) -> bool {
+        self.is_suppressing
+    }
+}
+
+/// Generates low-level comfort noise to fill silent periods.
+///
+/// When the decoder receives a comfort-noise descriptor (or detects a gap
+/// caused by silence suppression), it uses this to produce a natural-sounding
+/// background hiss instead of dead silence.
+pub struct ComfortNoise {
+    /// Peak amplitude of the generated noise (default: 50).
+    level: i16,
+}
+
+impl ComfortNoise {
+    /// Create a comfort noise generator with the given amplitude level.
+    pub fn new(level: i16) -> Self {
+        Self { level }
+    }
+
+    /// Fill `pcm` with low-level random noise in the range `[-level, level]`.
+    pub fn generate(&self, pcm: &mut [i16]) {
+        let mut rng = rand::thread_rng();
+        for sample in pcm.iter_mut() {
+            *sample = rng.gen_range(-self.level..=self.level);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn silence_detector_detects_silence() {
+        let mut det = SilenceDetector::new(100.0, 5);
+        let silence = vec![0i16; 960];
+
+        // First 5 frames are hangover — should NOT suppress yet.
+        for _ in 0..5 {
+            assert!(!det.is_silent(&silence));
+        }
+        // Frame 6 onward: past hangover, should suppress.
+        assert!(det.is_silent(&silence));
+        assert!(det.is_silent(&silence));
+    }
+
+    #[test]
+    fn silence_detector_detects_speech() {
+        let mut det = SilenceDetector::new(100.0, 5);
+
+        // Generate a 1kHz sine wave at decent amplitude.
+        let pcm: Vec<i16> = (0..960)
+            .map(|i| {
+                let t = i as f64 / 48000.0;
+                (10000.0 * (2.0 * std::f64::consts::PI * 1000.0 * t).sin()) as i16
+            })
+            .collect();
+
+        // Should never report silent.
+        for _ in 0..20 {
+            assert!(!det.is_silent(&pcm));
+        }
+    }
+
+    #[test]
+    fn silence_detector_hangover() {
+        let mut det = SilenceDetector::new(100.0, 3);
+        let silence = vec![0i16; 960];
+        let speech: Vec<i16> = (0..960)
+            .map(|i| {
+                let t = i as f64 / 48000.0;
+                (5000.0 * (2.0 * std::f64::consts::PI * 440.0 * t).sin()) as i16
+            })
+            .collect();
+
+        // Feed silence past hangover to enter suppression.
+        for _ in 0..4 {
+            det.is_silent(&silence);
+        }
+        assert!(det.is_silent(&silence), "should be suppressing after hangover");
+
+        // Speech arrives — should immediately stop suppressing.
+        assert!(!det.is_silent(&speech));
+        assert!(!det.is_silent(&speech));
+    }
+
+    #[test]
+    fn comfort_noise_generates_nonzero() {
+        let cn = ComfortNoise::new(50);
+        let mut pcm = vec![0i16; 960];
+        cn.generate(&mut pcm);
+
+        // At least some samples should be non-zero.
+        assert!(pcm.iter().any(|&s| s != 0), "CN output should not be all zeros");
+
+        // All samples should be within [-50, 50].
+        assert!(pcm.iter().all(|&s| s.abs() <= 50), "CN samples out of range");
+    }
+
+    #[test]
+    fn rms_calculation() {
+        // All zeros → RMS 0.
+        assert_eq!(SilenceDetector::rms(&[0i16; 100]), 0.0);
+
+        // Constant value: RMS of [v, v, v, ...] = |v|.
+        let pcm = vec![100i16; 100];
+        let rms = SilenceDetector::rms(&pcm);
+        assert!((rms - 100.0).abs() < 0.01, "RMS of constant 100 should be 100, got {rms}");
+
+        // Known pattern: [3, 4] → sqrt((9+16)/2) = sqrt(12.5) ≈ 3.5355
+        let rms2 = SilenceDetector::rms(&[3, 4]);
+        assert!((rms2 - 3.5355).abs() < 0.01, "RMS of [3,4] should be ~3.5355, got {rms2}");
+
+        // Empty buffer → 0.
+        assert_eq!(SilenceDetector::rms(&[]), 0.0);
+    }
+}