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).

custom_components/houseplan/validation.py

@@ -100,6 +100,9 @@ SPACE_SCHEMA = vol.Schema(
         vol.Required("aspect"): vol.All(vol.Coerce(float), vol.Range(min=0.05, max=20)),
         vol.Required("view_box"): vol.All([vol.Coerce(float)], vol.Length(min=4, max=4)),
         vol.Required("rooms"): [ROOM_SCHEMA],
+        # Legacy: walls are derived from room outlines since v1.19.0 — a line has no
+        # independent existence. Still accepted so a stale browser tab cannot fail a save;
+        # the card strips the field on every write.
         vol.Optional("segments"): [vol.All([vol.Coerce(float)], vol.Length(min=4, max=4))],
     },
     extra=vol.ALLOW_EXTRA,