fix: junction limits are honest at the boundaries (#331)

Six normative cuts, both mirrors symmetric (spec revision 3):

- §2.1 node keys quantise to 1e-7 with the repository's canonicalisation
  formula (sign·floor(|v|·1e7+0.5)/1e7, -0 normalised) — toFixed(6) keys
  split one node into two on floating debris and produced two false П4
  refusals on a legitimate resize (reproduced: -1e-8 vs 0). Node pairs
  within 2e-7 of each other (raw coordinates) are ONE node, and the
  node-to-wall incidence uses the same quantum.
- §2.2 a ~0° wedge IS a violation: two rays leaving a node the same way are
  a duplicated or overlaid wall (a butt joint yields 180°, never 0°) — the
  worst degenerate case was invisible while 0.5° was refused.
- §2.3/§2.4 the wall run is an iterative edge walk over the collinear
  component: no recursion (10 000 atoms answered, not RangeError), no
  silently dropped fork (the old .find lost every branch but the first),
  O(E) by construction, and collinearity is measured against the BASE
  segment's axis so an arc of 0.9°-per-atom pieces cannot pose as one wall.
- §2.5 an exception while judging the CANDIDATE refuses the write with the
  junction.limit_check_failed toast (fail-closed, as the #278 guard); the
  baseline branch stays fail-open by design and the smoke proves the
  asymmetry by breaking only the second call of the deterministic pair.
- §2.6 the python mirror narrows its except on the candidate side only:
  a genuine migration bug (TypeError) surfaces as an honest WS error, while
  a previous-side bug keeps the wide "no baseline" fallback — the two AC6
  cases pin the asymmetry so swapped sides turn a unit red.

Parity fixtures gain the new boundary classes (debris node, duplicate wall,
collinear fork); four new mutants pin the filter, the key precision, the
dropped branch and the fail-open hole.

Issue: #331
User-Visible: yes

src/i18n/en.json

@@ -589,6 +589,7 @@
   "junction.limit_length": "The wall segment is too short: {actual} cm, minimum {limit} cm.",
   "junction.limit_distance": "Walls and nodes are too close: {actual} cm, minimum {limit} cm.",
   "junction.limit_clearance": "No room interior is left: {actual} cm², minimum {limit} cm².",
+  "junction.limit_check_failed": "The junction check could not run — the change was not saved",
   "toast.wall_model_migration_blocked": "The wall model could not be updated: {reason}. The plan was not changed. Run “Optimize plans”; if the error repeats, fix the conflicting wall geometry.",
   "toast.wall_model_client_outdated": "Update the card and reload the page before editing the plan.",
   "wall_model.reason.invalid-room": "invalid room contour",

src/i18n/ru.json

@@ -589,6 +589,7 @@
   "junction.limit_length": "Слишком короткий участок стены: {actual} см, минимум {limit} см.",
   "junction.limit_distance": "Стены и узлы слишком близко: {actual} см, минимум {limit} см.",
   "junction.limit_clearance": "Внутри комнаты не остаётся места: {actual} см², минимум {limit} см².",
+  "junction.limit_check_failed": "Проверку стыков выполнить не удалось — изменение не сохранено",
   "toast.wall_model_migration_blocked": "Не удалось обновить модель стен: {reason}. План не изменён. Запустите «Оптимизировать планы»; если ошибка повторится, исправьте конфликтующую геометрию стен.",
   "toast.wall_model_client_outdated": "Обновите карточку и перезагрузите страницу перед редактированием плана.",
   "wall_model.reason.invalid-room": "некорректный контур комнаты",