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).

src/i18n/ru.json

@@ -27,13 +27,11 @@
   "title.configure_space": "Настроить пространство",
   "title.add_space": "Добавить пространство",
   "title.markup_add": "Добавить комнату: соединяйте точки сетки линиями до замкнутого контура",
-  "title.markup_erase": "Стереть линию: клик по линии",
   "title.markup_delroom": "Удалить комнату: клик внутри комнаты",
   "title.no_area_room": "Декоративная комната без привязки к зоне (например, холл)",
   "title.choose_area": "Выберите зону Home Assistant",
   "title.need_plan": "Загрузите подложку (план этажа)",
   "markup.add": "Добавить",
-  "markup.erase": "Стереть",
   "markup.delete": "Удалить",
   "markup.hint_points": "точек: {n} · Esc/Ctrl+Z — убрать точку · замкните контур кликом по первой",
   "markup.hint_start": "кликните точку сетки, чтобы начать контур",