Adds the Going-Medieval / Rimworld "door replaces wall" convention.
Painting a door on a tile occupied by a Wall (ghost OR completed):
* Reverses Wall._complete: erases the wall_layer stamp, marks the
pathfinder cell walkable, triggers room recompute.
* queue_free's the wall entity.
* Spawns the door ghost in its place via the normal designation flow.
Source of truth for "is there a wall here?" is World.build_queue, so the
rule covers both designation-painted walls and pre-built seeds (cabin,
test shed) which self-register via Wall._ready but aren't in
_build_sites_by_tile.
Verified via MCP: completed wall + door paint → wall gone, door ghost,
tile walkable, layer unstamped. Ghost wall + door paint → wall replaced
cleanly with no leftover ghosts.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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