fix IDXR invocation and minor bugs

fix IDXR invocation and minor bugs

- IDXR was called with the wrong endianness, and the hi-byte of notes_ix
  is now properly cleared. also, we use one less SRAM location.
- in the big multiplication in g(), we accidentally used SRC instead of
  SR in some places. by pure chance the bug wasn't triggered.
- some space is now left after the interrupt vector, so new code can be
  appended to the end, and the GOTO programmed 'over' with a NOP.
- fix some typos in comments.

this gets the music mostly working; there are some wrong notes, but it
is recongizable.