refactor(29-01): stabilize useMixerHelper function references

- Wrap getMixer with useCallback to ensure stable reference
- Prevents cascade of function recreations on every render
- Dependencies: mixMode (selector value)
- Uses allMixersRef.current (ref, not dependency)

This stabilization is prerequisite for effective context memoization.
All dependent functions (fillTrackVolumeObject, mute, faderChanged, etc.)
now automatically stabilize because their getMixer dependency is stable.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

jam-ui/src/hooks/useMixerHelper.js

@@ -125,13 +125,13 @@ const useMixerHelper = () => {
     // console.log("_XDEBUG_ useMixerHelper: allMixersRef updated", allMixersRef.current);
   }, [allMixers]);
 
-  const getMixer = (mixerId, mode) => {
+  const getMixer = useCallback((mixerId, mode) => {
     // Only default to mixMode if mode is undefined, not if it's explicitly false
     if (mode === undefined) {
       mode = mixMode;
     }
     return allMixersRef.current[(mode ? 'M' : 'P') + mixerId];
-  }
+  }, [mixMode]);
 
   const fillTrackVolumeObject = useCallback((mixerId, mode, currentAllMixers, broadcast = true) => {
     const mixer = getMixer(mixerId, mode, currentAllMixers);