docs: recapture gallery media and sync all index pages

docs: recapture gallery media and sync all index pages

- Replace legacy .webm/.png captures with '<name> - Screencast N.mp4'
  and '<name> - Screenshot N.png' files across 2D GFX, 3D GFX, Games,
  Math, Miscellaneous, Networking and Tutorial
- Point every index.org video/image reference at the new captures;
  sections whose screenshots were recaptured as screencasts now embed
  video blocks
- Fix broken source links: Bouncing cubes wrong directory prefix,
  copy-pasted '!.bas' links in Galaxy/Universe explorer, Alarm comma
  filenames, Pomppu Paavo moved into its own subdirectory
- Add 'Back to BASIC applications collection' parent link to all 13
  topic pages
- worm.bas: guard tail/head ring-buffer pointers against out-of-range
  access for dead or overgrown worms
- Stars.bas: drop spurious totalStars increment in galaxy creation
- Anaglyph.bas, Tiled room.bas: emit SOUND 0, .5 at main loop start