The single-row painter became vml.s at $C000: one sys() paints all 24
rows, memmove and line-index fix-ups run in ML, and the keyboard buffer
is popped with IRQs masked. dall() repaints only the cursor row or the
whole screen, so j went from 17 to 0-3 jiffies. Loading now checks the
drive error channel and splits lines over 80 columns. The temporary
jiffy counters used to measure the speedup are removed; the numbers are
in NOTES.md.

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