feat: jitter buffer instrumentation — drift test, telemetry, parameter sweep

WZP-P2-T1-S1: Automated drift measurement
- New drift_test.rs: DriftTestConfig, DriftResult, run_drift_test()
- CLI --drift-test <secs>: sends tone, measures actual vs expected duration
- Interpretation tiers: EXCELLENT (<50ms) / GOOD / FAIR / POOR
- 2 unit tests: drift math verification, config defaults

WZP-P2-T1-S2: Jitter buffer telemetry
- JitterStats gains: total_decoded, underruns, overruns, max_depth_seen
- JitterBuffer: record_underrun(), record_decode(), reset_stats()
- CallDecoder: stats() getter, reset_stats()
- JitterTelemetry: periodic tracing::info! logger with configurable interval
- 4 unit tests: ingestion tracking, underrun tracking, reset, interval

WZP-P2-T1-S3: Parameter sweep
- New sweep.rs: SweepConfig, SweepResult, run_local_sweep()
- Tests 20 jitter buffer configs (5 target × 4 max depths) locally
- CLI --sweep: runs sweep, prints ASCII comparison table
- No network needed — pure encoder→decoder pipeline test
- 3 unit tests: config defaults, local sweep runs, table formatting

216 tests passing across all crates.

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

crates/wzp-client/src/call.rs

@@ -2,8 +2,10 @@
 //!
 //! Pipeline: mic → encode → FEC → encrypt → send / recv → decrypt → FEC → decode → speaker
 
+use std::time::{Duration, Instant};
+
 use bytes::Bytes;
-use tracing::{debug, warn};
+use tracing::{debug, info, warn};
 
 use wzp_fec::{RaptorQFecDecoder, RaptorQFecEncoder};
 use wzp_proto::jitter::{JitterBuffer, PlayoutResult};
@@ -323,25 +325,37 @@ impl CallDecoder {
     pub fn decode_next(&mut self, pcm: &mut [i16]) -> Option<usize> {
         match self.jitter.pop() {
             PlayoutResult::Packet(pkt) => {
-                match self.audio_dec.decode(&pkt.payload, pcm) {
+                let result = match self.audio_dec.decode(&pkt.payload, pcm) {
                     Ok(n) => Some(n),
                     Err(e) => {
                         warn!("decode error: {e}, using PLC");
                         self.audio_dec.decode_lost(pcm).ok()
                     }
+                };
+                if result.is_some() {
+                    self.jitter.record_decode();
                 }
+                result
             }
             PlayoutResult::Missing { seq } => {
                 // Only generate PLC if there are still packets buffered ahead.
                 // Otherwise we've drained everything — return None to stop.
                 if self.jitter.depth() > 0 {
                     debug!(seq, "packet loss, generating PLC");
-                    self.audio_dec.decode_lost(pcm).ok()
+                    let result = self.audio_dec.decode_lost(pcm).ok();
+                    if result.is_some() {
+                        self.jitter.record_decode();
+                    }
+                    result
                 } else {
+                    self.jitter.record_underrun();
                     None
                 }
             }
-            PlayoutResult::NotReady => None,
+            PlayoutResult::NotReady => {
+                self.jitter.record_underrun();
+                None
+            }
         }
     }
 
@@ -351,8 +365,54 @@ impl CallDecoder {
     }
 
     /// Get jitter buffer statistics.
-    pub fn jitter_stats(&self) -> wzp_proto::jitter::JitterStats {
-        self.jitter.stats().clone()
+    pub fn stats(&self) -> &wzp_proto::jitter::JitterStats {
+        self.jitter.stats()
+    }
+
+    /// Reset jitter buffer statistics counters.
+    pub fn reset_stats(&mut self) {
+        self.jitter.reset_stats();
+    }
+}
+
+/// Periodic telemetry logger for jitter buffer statistics.
+///
+/// Call `maybe_log` on each decode tick; it will emit a `tracing::info!` event
+/// no more frequently than the configured interval.
+pub struct JitterTelemetry {
+    interval: Duration,
+    last_report: Instant,
+}
+
+impl JitterTelemetry {
+    /// Create a new telemetry logger that reports at most once per `interval_secs`.
+    pub fn new(interval_secs: u64) -> Self {
+        Self {
+            interval: Duration::from_secs(interval_secs),
+            last_report: Instant::now(),
+        }
+    }
+
+    /// Log jitter statistics if the interval has elapsed. Returns `true` when a
+    /// log line was emitted.
+    pub fn maybe_log(&mut self, stats: &wzp_proto::jitter::JitterStats) -> bool {
+        let now = Instant::now();
+        if now.duration_since(self.last_report) >= self.interval {
+            info!(
+                buffer_depth = stats.current_depth,
+                underruns = stats.underruns,
+                overruns = stats.overruns,
+                late_packets = stats.packets_late,
+                total_received = stats.packets_received,
+                total_decoded = stats.total_decoded,
+                max_depth_seen = stats.max_depth_seen,
+                "jitter buffer telemetry"
+            );
+            self.last_report = now;
+            true
+        } else {
+            false
+        }
     }
 }
 
@@ -558,4 +618,101 @@ mod tests {
         assert_eq!(catastrophic.profile, QualityProfile::CATASTROPHIC);
         assert!(catastrophic.jitter_max > degraded.jitter_max);
     }
+
+    // ---- JitterStats telemetry tests ----
+
+    fn make_test_packet(seq: u16) -> MediaPacket {
+        MediaPacket {
+            header: MediaHeader {
+                version: 0,
+                is_repair: false,
+                codec_id: CodecId::Opus24k,
+                has_quality_report: false,
+                fec_ratio_encoded: 0,
+                seq,
+                timestamp: seq as u32 * 20,
+                fec_block: 0,
+                fec_symbol: seq as u8,
+                reserved: 0,
+                csrc_count: 0,
+            },
+            payload: Bytes::from(vec![0u8; 60]),
+            quality_report: None,
+        }
+    }
+
+    #[test]
+    fn stats_track_ingestion() {
+        let config = CallConfig::default();
+        let mut dec = CallDecoder::new(&config);
+
+        for i in 0..5u16 {
+            dec.ingest(make_test_packet(i));
+        }
+
+        let stats = dec.stats();
+        assert_eq!(stats.packets_received, 5);
+        assert_eq!(stats.current_depth, 5);
+        assert_eq!(stats.max_depth_seen, 5);
+    }
+
+    #[test]
+    fn stats_track_underruns() {
+        let config = CallConfig::default();
+        let mut dec = CallDecoder::new(&config);
+
+        // Empty buffer — decode_next should record underruns
+        let mut pcm = vec![0i16; 960];
+        dec.decode_next(&mut pcm);
+        dec.decode_next(&mut pcm);
+        dec.decode_next(&mut pcm);
+
+        assert_eq!(dec.stats().underruns, 3);
+    }
+
+    #[test]
+    fn stats_reset() {
+        let config = CallConfig::default();
+        let mut dec = CallDecoder::new(&config);
+
+        // Generate some stats: ingest packets and trigger underruns on empty buffer
+        for i in 0..3u16 {
+            dec.ingest(make_test_packet(i));
+        }
+        // Also call decode on empty decoder to get underruns
+        let config2 = CallConfig::default();
+        let mut dec2 = CallDecoder::new(&config2);
+        let mut pcm = vec![0i16; 960];
+        dec2.decode_next(&mut pcm); // underrun — nothing in buffer
+
+        assert!(dec.stats().packets_received > 0);
+        assert!(dec2.stats().underruns > 0);
+
+        // Test reset on the decoder with ingested packets
+        dec.reset_stats();
+        let stats = dec.stats();
+        assert_eq!(stats.packets_received, 0);
+        assert_eq!(stats.underruns, 0);
+        assert_eq!(stats.overruns, 0);
+        assert_eq!(stats.total_decoded, 0);
+        assert_eq!(stats.packets_late, 0);
+        assert_eq!(stats.max_depth_seen, 0);
+
+        // Test reset on the decoder with underruns
+        dec2.reset_stats();
+        assert_eq!(dec2.stats().underruns, 0);
+    }
+
+    #[test]
+    fn telemetry_respects_interval() {
+        use wzp_proto::jitter::JitterStats;
+
+        let mut telemetry = JitterTelemetry::new(60); // 60-second interval
+        let stats = JitterStats::default();
+
+        // First call right after creation — should not log because no time has passed
+        // (the interval hasn't elapsed since construction)
+        let logged = telemetry.maybe_log(&stats);
+        assert!(!logged, "should not log before interval elapses");
+    }
 }