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/styles.ts

@@ -538,9 +538,21 @@ export const cardStyles = css`
     /* room-picking stages: merge (both clicks) and split before a room is chosen */
     .stage.markup.tool-merge,
     .stage.markup.tool-split.pickstage,
+    .stage.markup.tool-openwall,
     .stage.markup.tool-delroom {
       cursor: pointer;
     }
+    .openwall {
+      stroke: var(--card-background-color, var(--hp-bg, #fff));
+      stroke-width: 3.2;
+      stroke-dasharray: 7 7;
+      pointer-events: none;
+      opacity: 0.9;
+    }
+    .openwalls.hot .openwall {
+      stroke: #ffc14d;
+      opacity: 1;
+    }
     .stage.markup .room {
       pointer-events: none;
     }