Fix MP3/WAV export crash + harden audio backend

Reproduced with Puppeteer:

• Export crashed with "memory access out of bounds" at csoundSetOption, preceded by "Csound Command ERROR: no outfilename".

• renderEnded never fires and start() may not resolve at score end, so offline render now polls the output file until it stops growing, then stop()s to finalize the WAV header and reads it.

• Playback hardening for plain HTTPS hosts (no COOP/COEP): use the ScriptProcessor backend (useSPN) with our resumed AudioContext

▶ Play it: Food Court Eternity → Smoke Arithmetic → Barometric Stampede — a constellation across three families — the path is the mix https://stellate.app/?genre=vaporwave&seed=3090&path=1679.4415,292.16891,1718.20526&xdur=0.046

3eec0c8 https://github.com/aboard-io/stellate/commit/3eec0c829b231afeaf0ed4ecfc17c9e0330ea21a