audio: extract apply_peer_volume seam + A24 regression tests

A24 (per-peer volume slider has no effect) does not reproduce on current
main from a code trace: the UI slider's EndpointId is the same key the
mixer uses for the incoming jitter frame, and the gain is applied before
EQ/pan/output. Extract the inline per-peer lookup into a pure
apply_peer_volume() seam and add two regression tests:
 - matching key scales the frame (0.5 halves it)
 - mismatched key defaults to unity (guards the key-identity failure mode)

No wire/gossip/identity/PeerState change. The field-reported A24 was most
likely a stale listener build (volume is listener-side); needs a 2-machine
audible re-verify to close.

Co-Authored-By: Codex (gpt-5.5) <noreply@openai.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/mod.rs

@@ -294,6 +294,17 @@ fn apply_volume(frame: &mut [i16], vol: f32) {
     }
 }
 
+/// Apply the listener's per-peer volume for the audio sender id currently being
+/// mixed. The map key must be the same `EndpointId` used for the jitter buffer.
+fn apply_peer_volume(
+    frame: &mut [i16],
+    peer_id: EndpointId,
+    volumes: &HashMap<EndpointId, f32>,
+) {
+    let vol = volumes.get(&peer_id).copied().unwrap_or(1.0);
+    apply_volume(frame, vol);
+}
+
 /// Normalized RMS level of a frame in `[0.0, 1.0]` (32768 = full scale), for the
 /// UI level meter. An empty frame reads as 0.0.
 fn frame_level(frame: &[i16]) -> f32 {
@@ -1707,8 +1718,7 @@ async fn run_core_loop(
                                     peer_noise_gates.remove(&peer_id);
                                 }
 
-                                let vol = current_volumes.get(&peer_id).copied().unwrap_or(1.0);
-                                apply_volume(&mut frame, vol);
+                                apply_peer_volume(&mut frame, peer_id, &current_volumes);
 
                                 let eq_settings = current_eq
                                     .get(&peer_id)
@@ -2776,9 +2786,9 @@ async fn run_core_loop(
 #[cfg(test)]
 mod tests {
     use super::{
-        admit_retained, apply_volume, audio_datagram_len_ok, frame_level, mix_frames,
-        mix_stereo_frames, next_game_change, should_auto_fetch, stereo_to_mono, KnownPeers,
-        MicLevelMeter, PeerSpeakTicket, MAX_OPUS_PAYLOAD, MAX_RETAINED_PEERS,
+        admit_retained, apply_peer_volume, apply_volume, audio_datagram_len_ok, frame_level,
+        mix_frames, mix_stereo_frames, next_game_change, should_auto_fetch, stereo_to_mono,
+        KnownPeers, MicLevelMeter, PeerSpeakTicket, MAX_OPUS_PAYLOAD, MAX_RETAINED_PEERS,
         MIC_LEVEL_REPORT_SAMPLES,
     };
 
@@ -3038,6 +3048,30 @@ mod tests {
         assert_eq!(frame, vec![2000, -2000]);
     }
 
+    #[test]
+    fn peer_volume_map_scales_the_matching_audio_peer_frame() {
+        let peer = iroh::SecretKey::generate().public();
+        let other_peer = iroh::SecretKey::generate().public();
+        let volumes = std::collections::HashMap::from([(peer, 0.5), (other_peer, 2.0)]);
+        let mut frame = vec![100, -200, 300, -400];
+
+        apply_peer_volume(&mut frame, peer, &volumes);
+
+        assert_eq!(frame, vec![50, -100, 150, -200]);
+    }
+
+    #[test]
+    fn peer_volume_map_defaults_to_unity_when_audio_peer_key_is_unmatched() {
+        let ui_peer = iroh::SecretKey::generate().public();
+        let audio_peer = iroh::SecretKey::generate().public();
+        let volumes = std::collections::HashMap::from([(ui_peer, 0.5)]);
+        let mut frame = vec![100, -200, 300, -400];
+
+        apply_peer_volume(&mut frame, audio_peer, &volumes);
+
+        assert_eq!(frame, vec![100, -200, 300, -400]);
+    }
+
     #[test]
     fn three_peers_sum_without_saturation() {
         let a = vec![10, 20];