Avatars for the PWA.
* avatar-kit.js, avatars.json (mannequin variants + the eight cast characters), glb/mannequin_<build>_<cm>.glb (nine variants),
  glb/mannequin_clips.glb (the 17 shared clips: walk, walk_slow, walk_fast, walk_back, strafe_left, strafe_right, ...): the CAST-04 kit from assets/avatars/ (avatar-kit.js is copied; avatars.json is
  assets/avatars/avatars.json plus one variant per character, written by tools/optimize/avatars.mjs manifest).
* glb/char_<id>.glb: the eight sculpted cast characters, built by assets/avatars/characters/build_cast.sh (CharMorph + the CC0
  Vitruvian base, garments and hair authored per person; see assets/avatars/README.md and plan/reviews/avatars/).
The reference body mannequin_base.glb is not shipped.  Original work; Vitruvian base CC0.
