test(audio): regression tests for mic meter + gate drag

Extracts the peak-hold/throttle logic shared by the in-call capture thread
and run_mic_monitor into MicLevelMeter, and adds unit coverage:

- core: MicLevelMeter reports only after a full window, holds the window
  peak, resets between windows, and reports zero for silence.
- app: GateMeter::x_to_threshold maps edges/midpoint correctly, clamps
  out-of-bounds drags, and stays finite for a zero-width (pre-layout) bar.

9 new tests, all green; clippy clean.

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

src/app/mod.rs

@@ -1150,11 +1150,43 @@ impl Program<AppMessage> for GateMeter {
 
 #[cfg(test)]
 mod tests {
-    use super::{reconnect_attempt_chime, reconnected_chime};
+    use super::{reconnect_attempt_chime, reconnected_chime, GateMeter, METER_MAX};
     use crate::notify::Sound;
     use iroh::EndpointId;
     use std::collections::HashSet;
 
+    const W: f32 = 200.0;
+
+    #[test]
+    fn gate_drag_maps_left_edge_to_zero() {
+        assert_eq!(GateMeter::x_to_threshold(0.0, W), 0.0);
+    }
+
+    #[test]
+    fn gate_drag_maps_right_edge_to_full_scale() {
+        assert!((GateMeter::x_to_threshold(W, W) - METER_MAX).abs() < 1e-6);
+    }
+
+    #[test]
+    fn gate_drag_maps_midpoint_to_half_scale() {
+        assert!((GateMeter::x_to_threshold(W / 2.0, W) - METER_MAX / 2.0).abs() < 1e-6);
+    }
+
+    #[test]
+    fn gate_drag_clamps_out_of_bounds() {
+        // Dragging past either edge clamps to the axis ends (no overshoot).
+        assert_eq!(GateMeter::x_to_threshold(-50.0, W), 0.0);
+        assert!((GateMeter::x_to_threshold(W + 80.0, W) - METER_MAX).abs() < 1e-6);
+    }
+
+    #[test]
+    fn gate_drag_zero_width_is_finite() {
+        // A degenerate bound (pre-layout) must not divide by zero / produce NaN.
+        let t = GateMeter::x_to_threshold(10.0, 0.0);
+        assert!(t.is_finite());
+        assert!((0.0..=METER_MAX).contains(&t));
+    }
+
     /// A distinct, real `EndpointId` (via the same path the network tests use).
     fn id() -> EndpointId {
         iroh::EndpointAddr::from(iroh::SecretKey::generate().public()).id