feat v1.19.0: a line is never an entity of its own — walls are derived from rooms

A wall can only exist as an edge of a closed room:
- roomEdges(rooms) derives walls from room outlines, deduping shared ones, so deleting a
  room keeps the borders its neighbours still contribute and drops the rest — the rule
  falls out of the model instead of needing bookkeeping.
- Nothing is persisted while drawing: an outline you never close leaves no lines behind
  (previously every click pair was written to space.segments immediately).
- The 'Erase line' tool is gone; space.segments is stripped on every save (validation
  still tolerates it on read so a stale tab cannot fail a save).
- Dead code out: _addSegment/_removeSegmentByKey/_distToSeg/_pathSegs/_segKey.
Docs (ARCHITECTURE/TESTING/CHANGELOG) updated in the same commit. +2 tests (63).

docs/TESTING.md

@@ -63,7 +63,14 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
 
 ## Room markup editor ★
 
-- [ ] Grid appears; dots snap; segments draw pair-by-pair; shared walls reused
+- [ ] Grid appears; dots snap; the outline draws pair-by-pair; shared walls reused
+- [ ] Ruler: while drawing, the length of the current segment follows the cursor
+      (metres, or feet+inches on an imperial HA); scale = space "cm per cell" (default 5)
+- [ ] A line cannot exist on its own: start an outline, do NOT close it, leave markup —
+      no lines are left behind (nothing was written to the config)
+- [ ] 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)
 - [ ] 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