perf/fix v1.43.1: external audit P1 — render cost, drag threshold, geometry, backend

L1: memoized space model + open pairs (structural fingerprint key, epoch
bumped synchronously at mutation time, not inside the debounce); hoisted
per-room geometry out of the render loop; smoke asserts zero recomputation
across state pushes.

L4: openings get the 3 px drag threshold used by every other pipeline and
only write when the geometry actually changed — taps open the dialog again.

G2: interiorPoint() replaces the vertex mean, so island rooms inside
concave (U/L) parents are accepted and their evenodd holes render; traced
duplicates still are not containment.

G3: segKey rounds before ordering — one shared wall, one key.

B2: _check_write fails closed when the entry is unavailable.
B3: layout/set honours expected_rev and returns the new rev.
B4: config/set without expected_rev over a non-empty store logs a warning.
B5: coordinates reject NaN/Infinity; spaces/rooms/markers/decor/layout capped.

+2 unit tests (118), +2 backend tests (14), smoke_render_perf; docs
same-commit

demo/smoke_save_race.mjs

@@ -21,7 +21,7 @@ const res = await page.evaluate(async () => {
   const sp = c._curSpaceCfg;
   sp.rooms.push({ id: 'race_room', name: 'RACE', area: null, poly: [[0.8, 0.8], [0.9, 0.8], [0.9, 0.9], [0.8, 0.9]] });
   c._saveConfig();
-  out.pending = c._saveConfig.pending();
+  out.pending = c._saveConfigDebounced.pending();
   // через 100 мс приходит событие о чужой ревизии — раньше это стирало правку
   c._cfgRev = server.rev; // симулируем: наша ревизия отстала
   await c._reloadConfigOnly();