test(core): mixer edge cases — volume rounding, multi-peer, frame_level

Extend the mixer unit tests: pin apply_volume's as-i16 truncation-toward-zero
semantics (3*0.5 -> 1, -3*0.5 -> -1, not round-to-nearest), in-range amplify
without clamp, three-peer sum, mix_frames zero-padding when the output is
longer than every peer frame, and frame_level mid-range (constant 16384 ->
~0.5). Tests-only; no production 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/core/mod.rs

@@ -892,4 +892,42 @@ mod tests {
         let level = frame_level(&full);
         assert!(level > 0.99 && level <= 1.0, "full-scale level was {level}");
     }
+
+    #[test]
+    fn volume_truncates_toward_zero() {
+        let mut frame = vec![3, -3, 5, -5];
+        apply_volume(&mut frame, 0.5);
+        assert_eq!(frame, vec![1, -1, 2, -2]);
+    }
+
+    #[test]
+    fn volume_amplifies_without_saturating() {
+        let mut frame = vec![1000, -1000];
+        apply_volume(&mut frame, 2.0);
+        assert_eq!(frame, vec![2000, -2000]);
+    }
+
+    #[test]
+    fn three_peers_sum_without_saturation() {
+        let a = vec![10, 20];
+        let b = vec![3, 4];
+        let c = vec![100, -50];
+        let mixed = mix_frames(&[a, b, c], 2);
+        assert_eq!(mixed, vec![113, -26]);
+    }
+
+    #[test]
+    fn mix_zero_pads_output_longer_than_peer_frames() {
+        let a = vec![100, 200];
+        let mixed = mix_frames(&[a], 4);
+        assert_eq!(mixed, vec![100, 200, 0, 0]);
+    }
+
+    #[test]
+    fn frame_level_mid_range() {
+        let frame = vec![16384; 64];
+        let level = frame_level(&frame);
+        assert!((level - 0.5).abs() < 1e-3, "mid-range level was {level}");
+    }
 }
+