fix(SandboxSynth): pass index to unison voices forEach

[espadonne]

Summary

• voices.forEach(voice => {...}) at components/audio/InstrumentSandbox/SandboxSynth.js:782 was missing the index argument, but the body still referenced i from an earlier for (let i...) loop used to build the voices array.
• Result: ReferenceError: i is not defined when starting a unison voice — reproduced by a tester entering Arco from the Studio modal.
• Fix takes the index directly from the forEach callback. Inner for (let i...) loops in the body each declare their own i, so no shadowing.

Worth manually verifying: open the Studio modal, select Arco, play a note, and confirm no runtime error. With unisonVoices > 1, check that the [SandboxSynth] Started voice N (k/numVoices) log lines report correct indices.