test(codec): unit tests for Opus encode/decode + PLC sizing

Add #[cfg(test)] coverage for src/codec/opus_impl.rs: encode→decode
round-trip shape + signal-energy survival, decoded duration follows the
packet, and the key regression guard — decode(None)/decode(Some(&[]))
conceals exactly frame_samples per channel (960 mono / 1920 stereo),
pinning the previously-fixed 120ms-burst PLC sizing bug. Tests-only; no
production behavior change.

Implemented by Gemini (junior implementer), reviewed and verified by senior
(cargo build + clippy --all-targets + cargo test all green).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/codec/opus_impl.rs

@@ -80,3 +80,80 @@ impl AudioDecoder for OpusDecoder {
         Ok(pcm)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_round_trip() {
+        let mut encoder = OpusEncoder::new(48000, Channels::Mono, Application::Voip).unwrap();
+        let mut decoder = OpusDecoder::new(48000, Channels::Mono, 960).unwrap();
+
+        // 1. Round-trip shape: generate a 960-sample mono PCM frame (440Hz sine wave)
+        let mut pcm = vec![0i16; 960];
+        for (i, sample) in pcm.iter_mut().enumerate() {
+            let t = (i as f32) / 48000.0;
+            let val = (t * 440.0 * 2.0 * std::f32::consts::PI).sin();
+            *sample = (val * 10000.0) as i16;
+        }
+
+        // encode it
+        let compressed = encoder.encode(&pcm).unwrap();
+        assert!(!compressed.is_empty(), "Compressed buffer should not be empty");
+        assert!(
+            compressed.len() < pcm.len() * std::mem::size_of::<i16>(),
+            "Compressed size ({}) should be smaller than raw PCM size ({})",
+            compressed.len(),
+            pcm.len() * std::mem::size_of::<i16>()
+        );
+
+        // decode it
+        let decoded = decoder.decode(Some(&compressed)).unwrap();
+        assert_eq!(decoded.len(), 960, "Decoded sample count should be exactly 960");
+
+        // 2. Round-trip carries signal energy (not silence)
+        let sum_sq: f64 = decoded.iter().map(|&x| (x as f64).powi(2)).sum();
+        let rms = (sum_sq / decoded.len() as f64).sqrt();
+        // Since input had amplitude ~10000, let's verify RMS is significantly above 0 (e.g. > 100.0)
+        assert!(rms > 100.0, "Decoded signal should carry energy (RMS was {})", rms);
+    }
+
+    #[test]
+    fn test_plc_sizing() {
+        let mut decoder = OpusDecoder::new(48000, Channels::Mono, 960).unwrap();
+
+        // decode(None) returns exactly frame_samples (960) samples
+        let plc_none = decoder.decode(None).unwrap();
+        assert_eq!(plc_none.len(), 960, "decode(None) should yield exactly 960 samples");
+
+        // decode(Some(&[])) (empty slice) does the same
+        let plc_empty = decoder.decode(Some(&[])).unwrap();
+        assert_eq!(plc_empty.len(), 960, "decode(Some(&[])) should yield exactly 960 samples");
+    }
+
+    #[test]
+    fn test_decoded_duration_follows_packet() {
+        let mut encoder = OpusEncoder::new(48000, Channels::Mono, Application::Voip).unwrap();
+        let mut decoder = OpusDecoder::new(48000, Channels::Mono, 960).unwrap();
+
+        let pcm = vec![0i16; 960];
+        let compressed = encoder.encode(&pcm).unwrap();
+        let decoded = decoder.decode(Some(&compressed)).unwrap();
+        assert_eq!(decoded.len(), 960, "Decoded sample count should match packet duration");
+    }
+
+    #[test]
+    fn test_stereo_plc_sizing() {
+        let mut decoder = OpusDecoder::new(48000, Channels::Stereo, 960).unwrap();
+
+        // decode(None) returns exactly frame_samples * 2 (1920) samples
+        let plc_none = decoder.decode(None).unwrap();
+        assert_eq!(plc_none.len(), 960 * 2, "Stereo decode(None) should yield exactly 1920 samples");
+
+        // decode(Some(&[])) (empty slice) does the same
+        let plc_empty = decoder.decode(Some(&[])).unwrap();
+        assert_eq!(plc_empty.len(), 960 * 2, "Stereo decode(Some(&[])) should yield exactly 1920 samples");
+    }
+}
+