Episode: Tile assembly features (chamfered walls, alignment keys, underside mark)

EPISODE-GOAL

Chamfer the tile side walls so each wall’s plane passes through the globe center, add vertical fin/groove alignment keys on the mating walls, and engrave the tile id on the underside, so the tiles assemble into a globe without wedge gaps, print without supports, and are easy to identify and align.

EPISODE-PHASE

phase 1

EPISODE-ACCEPTANCE

• Does every wall strip lie in a plane through the globe center (in the pre-translation tile frame)?
• Do adjacent tiles share exactly one wall plane per mating edge, with vertical fins on the north/east walls mating vertical grooves on the south/west walls (polar caps grooved), so the assembled set has no wedge gaps and every fin meets a groove?
• Are the keys vertical (running from the top edge down the wall), so the tiles print without supports?
• Does the underside carry an engraved (recessed) tile id while the flat bottom stays at z = 0?
• Does make check still pass (watertight, flat bottom at z = 0, expected bounds, positive signed volume) on the rebuilt full-globe set?
• Is the whole change reviewable in one sitting?

EPISODE-RISKS

• Do the stud/socket features fit the thin (3-9 mm) wall strips — are the keys sized to the local wall height and skipped where the wall is too thin?
• Do the key and groove cut-outs keep every edge shared by exactly two faces (watertightness)?

EPISODE-TASKS

• Replace the vertical wall quads with chamfered strips in build_mesh: each strip is bounded above by the top-surface boundary edge and below by the radial projection of that edge onto the bottom plane, so the strip plane contains the globe center.
• Add stud/socket alignment keys: north and east walls carry studs, south and west walls carry sockets, polar-cap rings carry sockets per adjacent band tile; size each key to the local wall height and skip when too thin.
• Engrave the tile id on the underside with a minimal stroke font; keep z_min = 0 (recessed grooves, not raised marks).
• Extend the manifest and check mode for the new bottom-face structure (faces_bottom) and record the assembly features.
• Extend tests/04-stl-tiles-math.sh with chamfer-plane, key-convention, and underside-mark invariants.
• Rebuild the full-globe set and verify with make check and make test.

EPISODE-OUTPUT

Report the wall/key/mark geometry, the changed files, and the check/test results.

EPISODE-FILES

• scripts/stl_tiles_build.py — existing
• tests/04-stl-tiles-math.sh — existing
• documents/05-stl-approach.md — existing
• prompts/features/03-stl-tiles.md — existing
• README.md — existing
• TODO.md — existing

EPISODE-BRANCH

llm/episode-03