Add browser (Csound WASM) player + interactive song builder, with MP3 export

Run the capability in a browser tab — no Csound install:

• csd-engine.js — pure buildCsd(state) -> Csound text. Same engine as royal-road.csd, but the score is data-driven so a UI can rearrange it.

• wasm-audio.js — boots @csound/browser; decodes ANY audio URL via Web Audio into a WAV that ftgen can granulate (mp3/ogg/wav/m4a); realtime play + offline WAV render.

• wasm-ffmpeg.js — MP3 export via ffmpeg.wasm (single-thread core; no SharedArrayBuffer / COOP-COEP needed, runs on plain localhost).

• builder.html — full song builder: every knob, add audio by URL, export WAV / MP3 / .csd / preset JSON.

Verified: the generator emits valid Csound (13 native renders pass) and the builder UI initializes + generates CSD in a real (headless) browser.

▶ Play it: Ballroom Leviathan — a horn-and-piano genre that plays almost entirely sampled instruments https://stellate.app/?genre=bigband&seed=5329&path=814.14134,833.14167,795.14167&xdur=13.122

0ce7a7a https://github.com/aboard-io/stellate/commit/0ce7a7a9a102bd0ac4c83a6d45e0933c6fa1f2d5