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).

src/i18n/en.json

@@ -92,6 +92,8 @@
   "toast.markup_needs_server": "Markup is available after the config is moved to the server",
   "toast.conflict": "Config was changed in another window — data refreshed, repeat your last action",
   "toast.cfg_save_failed": "Failed to save config: {err}",
+  "toast.point_in_room": "That point is inside room “{name}” — rooms must not overlap",
+  "toast.room_overlap": "The outline overlaps room “{name}” — rooms must not overlap",
   "toast.room_saved": "Room saved ({n}). Devices added: {added}. Outline the next one or exit markup.",
   "toast.room_saved_no_area": "Room saved ({n}, no area). Outline the next one or exit markup.",
   "toast.marker_needs_server": "Device editing is available after the config is moved to the server",