refactor(29-01): memoize MixersContext provider value

- Add useMemo wrapper around mixerHelper value
- Prevents new object creation on every render
- Depends on stable mixerHelper (from Task 1a)
- Context consumers now only re-render when data actually changes

This eliminates cascading re-renders when unrelated state updates occur.

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

jam-ui/src/context/MixersContext.js

@@ -1,4 +1,4 @@
-import React, { createContext, useContext } from 'react';
+import React, { createContext, useContext, useMemo } from 'react';
 import useMixerHelper from '../hooks/useMixerHelper.js';
 
 const MixersContext = createContext();
@@ -6,8 +6,11 @@ const MixersContext = createContext();
 export const MixersProvider = ({ children }) => {
   const mixerHelper = useMixerHelper();
 
+  // Memoize context value to prevent unnecessary re-renders of consumers
+  const value = useMemo(() => mixerHelper, [mixerHelper]);
+
   return (
-    <MixersContext.Provider value={mixerHelper}>
+    <MixersContext.Provider value={value}>
       {children}
     </MixersContext.Provider>
   );