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).