refactor(audio): extract audio-stage settings into mix_commands

Pull the six configuration setters out of audio/commands.rs into
audio/mix_commands.rs (~170 LOC):
- audio_set_volume (with replay-gain-aware ramp)
- audio_update_replay_gain (resolves cache-backed loudness, computes
  gain, ramps sink, emits NormalizationStatePayload)
- audio_set_eq (10-band gains + pre-gain)
- audio_set_crossfade
- audio_set_gapless
- audio_set_normalization

These are pure AudioEngine state mutations + (for normalization) an
ipc emit. They don't drive playback; collecting them in one place
makes commands.rs more focused on the playback orchestrators.

lib.rs invoke_handler updated. audio/commands.rs: 1784 → 1614 LOC.

src-tauri/src/lib.rs

@@ -392,9 +392,9 @@ pub fn run() {
             audio::commands::audio_resume,
             audio::commands::audio_stop,
             audio::commands::audio_seek,
-            audio::commands::audio_set_volume,
-            audio::commands::audio_update_replay_gain,
-            audio::commands::audio_set_eq,
+            audio::mix_commands::audio_set_volume,
+            audio::mix_commands::audio_update_replay_gain,
+            audio::mix_commands::audio_set_eq,
             audio::autoeq_commands::autoeq_entries,
             audio::autoeq_commands::autoeq_fetch_profile,
             audio::commands::audio_preload,
@@ -403,9 +403,9 @@ pub fn run() {
             audio::preview::audio_preview_stop,
             audio::preview::audio_preview_stop_silent,
             audio::preview::audio_preview_set_volume,
-            audio::commands::audio_set_crossfade,
-            audio::commands::audio_set_gapless,
-            audio::commands::audio_set_normalization,
+            audio::mix_commands::audio_set_crossfade,
+            audio::mix_commands::audio_set_gapless,
+            audio::mix_commands::audio_set_normalization,
             audio::device_commands::audio_list_devices,
             audio::device_commands::audio_canonicalize_selected_device,
             audio::device_commands::audio_default_output_device_name,