feat: make SOUND/BEEP audible, add SPACE$ and LET, guard empty loops

feat: make SOUND/BEEP audible, add SPACE$ and LET, guard empty loops

- SoundQueue renders a phase-continuous square wave (44.1 kHz 16-bit)
  through a daemon-fed SourceDataLine in GUI mode; falls back to
  timing-only in headless/tests and when no audio device opens
- SPACE$ returns n spaces (previously misread as an array reference,
  surfacing as "Subscript out of range")
- LET keyword parses as plain scalar or array-element assignment
- empty-body DO/WHILE spins now enforce the --steps budget, so
  headless INKEY$ wait loops terminate instead of hanging