test(audio): edge-case unit tests for the noise gate

Covers the previously-untested branches of the NoiseGate envelope/timing:
frame_rms known values, empty-frame transmit-follows-state, disabled gate
parks the envelope open (no fade-in on re-enable), hold-window-then-release
ordering, sustained mid-level refreshes the hold, and a loud signal
re-opening a releasing gate. Gate tests 6 -> 12; test-only, no prod change.

Implemented by Gemini per next-task.md; reviewed against the real diff and
re-verified (build + clippy --all-targets + test all green) by the senior.

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

src/audio/gate.rs

@@ -227,4 +227,112 @@ mod tests {
         let mut f = frame(0);
         assert!(!g.process(&mut f, 0.05), "an unopened gate on silence must not transmit");
     }
+
+    #[test]
+    fn frame_rms_known_values() {
+        assert_eq!(frame_rms(&[]), 0.0);
+        assert_eq!(frame_rms(&frame(0)), 0.0);
+
+        let f_high = frame(16384);
+        let rms_high = frame_rms(&f_high);
+        assert!((rms_high - 0.5).abs() < 1e-4, "rms_high was {}", rms_high);
+
+        let f_low = frame(3277);
+        let rms_low = frame_rms(&f_low);
+        assert!((rms_low - 0.1).abs() < 1e-3, "rms_low was {}", rms_low);
+    }
+
+    #[test]
+    fn empty_frame_transmit_follows_gate_state() {
+        let mut g = NoiseGate::new(SR);
+        // fresh gate (never opened)
+        assert!(!g.process(&mut [], 0.05));
+
+        // open it with loud signals
+        for _ in 0..3 {
+            let mut f = frame(10000);
+            assert!(g.process(&mut f, 0.05));
+        }
+
+        // now empty frame should transmit
+        assert!(g.process(&mut [], 0.05));
+    }
+
+    #[test]
+    fn disabled_parks_envelope_so_reenable_has_no_fade_in() {
+        let mut g = NoiseGate::new(SR);
+        let mut f1 = frame(5000);
+        assert!(g.process(&mut f1, 0.0)); // disabled
+
+        let mut f2 = frame(10000);
+        assert!(g.process(&mut f2, 0.05)); // enabled
+        assert!(f2[0].abs() > 9000, "expected first sample of enabled frame to have no fade-in, got {}", f2[0]);
+    }
+
+    #[test]
+    fn hold_keeps_open_through_window_then_releases_to_closed() {
+        let mut g = NoiseGate::new(SR);
+        // open it
+        for _ in 0..3 {
+            let mut f = frame(10000);
+            g.process(&mut f, 0.05);
+        }
+
+        let mut results = Vec::new();
+        for _ in 0..25 {
+            let mut f = frame(0);
+            results.push(g.process(&mut f, 0.05));
+        }
+
+        assert!(results[4], "should still transmit at the 5th silent frame");
+        assert!(!results[19], "should not transmit at the 20th silent frame");
+    }
+
+    #[test]
+    fn sustained_mid_level_refreshes_hold_and_stays_open() {
+        let mut g = NoiseGate::new(SR);
+        // open it loud
+        for _ in 0..3 {
+            let mut f = frame(10000);
+            g.process(&mut f, 0.05);
+        }
+
+        // feed 30x frame(1310) (rms ~0.04, between close 0.03 and open 0.05)
+        for _ in 0..30 {
+            let mut f = frame(1310);
+            assert!(g.process(&mut f, 0.05));
+        }
+        assert!(g.open, "gate must stay open (hold refreshed by mid-level input)");
+    }
+
+    #[test]
+    fn loud_signal_reopens_a_releasing_gate() {
+        let mut g = NoiseGate::new(SR);
+        // open it
+        for _ in 0..3 {
+            let mut f = frame(10000);
+            g.process(&mut f, 0.05);
+        }
+
+        // feed silent frames to fully close
+        let mut closed = false;
+        for _ in 0..40 {
+            let mut f = frame(0);
+            if !g.process(&mut f, 0.05) {
+                closed = true;
+                break;
+            }
+        }
+        assert!(closed);
+
+        // feed frame(10000) @ 0.05 a few times
+        let mut last_peak = 0;
+        for _ in 0..3 {
+            let mut f = frame(10000);
+            assert!(g.process(&mut f, 0.05));
+            last_peak = peak(&f);
+        }
+        assert!(g.open);
+        assert!(last_peak >= 9900, "peak of the 3rd reopened frame must be >= 9900, got {}", last_peak);
+    }
 }