feat: irrational_music plugin + move api_test to project root

- Removed api_test from built-in plugins (saved to api_test.json)
- New 'irrational_music' fullscreen plugin:
  - Input any number expression (Pi, E, Sqrt[2], ...)
  - Generates up to 200 digits via IJS.Symja.numeric
  - Each digit 0-9 mapped to musical notes (C,D,E,F,G,A,B,C,D,E)
  - 4 instruments: sine, square, sawtooth, triangle
  - 4 chord modes: single, major triad, minor triad, power fifth
  - Adjustable tempo (60-300 bpm) and octave (3-6)
  - Scrolling digit display with current position highlight
  - Play/Pause/Stop controls
  - Music stops on panel close

Co-Authored-By: Claude <noreply@anthropic.com>