Fix export hang on "stop promise timed out" + empty MP3

csound stop()/destroy() can hang (the "stop promise timed out" warning); stopLive() and render()'s finalize now race the stop against a short timeout so export can't stall on teardown of the playing instance.

• MP3 came out ~0 bytes because Csound leaves the WAV data-chunk size field unfinalized; parseWav now falls back to the actual remaining byte length.

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015535f https://github.com/aboard-io/stellate/commit/015535fdc2b42cf6f99106c5af4759c096bf84a4