feat v1.37.0: open boundaries (virtual walls)

- 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

src/types.ts

@@ -1,6 +1,8 @@
 /** Shared types of the House Plan card. */
 
 export interface RoomCfg {
+  /** Rooms this one has an OPEN (virtual) boundary with — light flows through. */
+  open_to?: string[] | null;
   id?: string;
   name: string;
   area: string | null;