refactor(mouse-driver): slim qbext TSR, ungate programs, refresh media

refactor(mouse-driver): slim qbext TSR, ungate programs, refresh media

- qbext: 506 to 234 bytes — drop the unused update counter, inline the
  startup banner into the source, add build.sh; index.org rewritten to
  document the minimal 8-byte shared-memory protocol
- Galaxy/Universe/Maze explorer and Swapping 3D engine gated all mouse
  reads on the removed counter byte (getByte(8)) and silently froze
  with the new driver — drop the gate, read-and-reset every frame
- recapture showcase media as CRT Basic screencasts/screenshots
  (mp4/png replacing webm/png); rename Windowing system.bas to
  wsystem.bas