docs(showcase): asset sheets for the last 17 pieces; 76/76 now carry one

#202 asked for an asset sheet per showcase piece: the hero rendered alone
(neutral three-quarter view, grey sweep, no staging) beside the pinned
reference set, judged on one question: is the candidate the least-designed
object in the lineup? The harness (scripts/asset_sheet.py +
asset_sheet_panel.py) landed earlier; 17 pieces still had no SELECT row and
no sheet.

- scripts/asset_sheet.py: SELECT rows for bamboo-clump, cart, crate-stack,
  cricket-wicket, hay-bale, iron-cauldron, market-stall, moka-pot,
  park-bench, stone-archway, stone-well, sundial, toboggan, treasure-chest,
  wall-torch, wooden-barrel, wooden-bucket (each piece's LOD0 object).
- docs/gallery/asset-sheets/: the 17 sheets. Rendered with
  .scratch/blender-5.2.1-windows-x64 (5.2.1 LTS); references
  collision-hull-proxy, socket-attach-points, vertex-color-ao, wheelbarrow,
  apothecary-shelf. Not published (stage_public.py strips asset-sheets/).

Verdicts, by eye on each sheet:
- pass: bamboo-clump, cart, crate-stack, cricket-wicket, market-stall,
  park-bench, stone-well, sundial, toboggan, wooden-barrel, wooden-bucket
- weak pass: hay-bale, stone-archway (plain pale blocks), treasure-chest,
  wall-torch (flame is a crude cone)
- FAIL: iron-cauldron (a bare pot on three plain poles) and moka-pot
  (flat-shaded facets, untextured aluminium); each reads as the
  least-designed object on its sheet. Remodel tracked separately.

Closes #202

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: TMHSDigital