- room.open_to symmetric links; 'Open boundary' plan tool toggles the
shared wall pair under the click (sharedBoundary collinear-overlap
math + distToSegment pull, same as Split); dashed rendering with an
amber hot state while the tool is active; Esc -> Draw
- glow: the clip becomes the transitive open zone (openZoneOf BFS,
either-direction links) + door sectors from the zone's outer walls
- model rooms carry open_to; backend ROOM_SCHEMA open_to: [str]
- +3 unit tests (104), smoke_openwall.mjs (8 checks); docs same-commit
