feat v1.20.0: rooms may not overlap

- A click strictly inside an existing room is refused while drawing; a click ON a wall
  stays legal (neighbours share walls, and real walls overlap collinearly rather than
  match exactly, so vertices land on existing outlines mid-span constantly).
- Closing an outline that overlaps an existing room is refused — vertex checks alone are
  not enough, an outline drawn AROUND a room has every vertex outside it. Nesting counts
  as overlap. The outline stays open so it can be corrected.
- New pure geometry: roomPoly, pointOnBoundary, pointStrictlyInside, segmentsProperlyCross
  (touching/collinear deliberately not a crossing), roomsOverlap. +4 tests (67).
Verified live: all 15 existing neighbour pairs stay clean (no false positives).

docs/TESTING.md

@@ -71,6 +71,9 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
 - [ ] Deleting a room removes its walls, EXCEPT those shared with a neighbouring room
       (the neighbour still yields them); deleting the neighbour too removes them as well
 - [ ] There is no "Erase" tool in the markup toolbar (removed in v1.19.0)
+- [ ] Rooms never overlap (v1.20.0): a click strictly inside an existing room is refused with a
+      toast; a click ON a shared wall (including mid-span of a longer neighbour wall) still works
+- [ ] Closing an outline drawn AROUND an existing room is refused; the outline stays open
 - [ ] Esc / Ctrl+Z removes the last dot (and its line); Reset clears the path
 - [ ] Closing the contour (click the first dot, ≥4 points) opens the room dialog
 - [ ] Room dialog: area list shows only unassigned areas; picking an area prefills the name