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**Critical rule: the border terrain (typically Wall) must be shared across all families.** `Border=` and `Default=` can only reference one name. Splitting the border terrain per family silently breaks painting for some families.
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| Type | Shared or Split? | Reason |
|------|------------------|--------|
| Border terrain (Wall) | **Shared** | Required by Border=/Default=. Splitting breaks solver. |
| Floor, Pit, Subfloor, Platform Non-border terrains | Split per family | Family-specific visuals |
| All crossers | Split per family | Family-specific appearance |
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- [ ] Tile headers sequential 0 to N-1
- [ ] Models in sorted order
- [ ] `[TILES] Count=` matches actual count
- [ ] Door sections match parent tiles
- [ ] Tiles with `Doors > 0` have `DoorVisibilityNode` and `DoorVisibilityOrientation` (flag to user if missing)
- [ ] Group references point to valid tile indices
- [ ] `[TERRAIN TYPES] Count=` and `[CROSSER TYPES] Count=` match actual
- [ ] Every defined terrain/crosser used by at least one tile (and vice versa)
- [ ] Border terrain is shared (not split per family)
- [ ] `Border=` and `Default=` reference the shared border terrain
- [ ] Every .set model has matching .mdl and .wok (no orphans)
- [ ] ITP paint subfolders use `ID=2`
- [ ] ITP RESREFs match .set names