Builder: rotate to clip matrix, MIDI export, reseed/preview/playhead

Address UX feedback:

• Rotate the grid: sections are rows (time ↓), instruments are columns (Pads/Bass/Drums/Melody/Found), each color-coded.

• Remove the on-page Csound source viewer entirely.

• Playhead: highlights the currently-playing section row during playback (performance.now → beat → section), Ableton-style.

• Per-cell ▶ to audition just that layer/section.

▶ Play it: Rail Replacement Rapture — a bed-heavy genre where the found-sound layer carries the room https://stellate.app/?genre=transitwave&seed=5669&path=749.12536,768.12569,730.12569&xdur=13.122

43f063d https://github.com/aboard-io/stellate/commit/43f063d579618959579e7fac6dc037635a388951