fix v1.38.4: derived wall segments trimmed under open boundaries

- the markup layer's .seg lines ran solid through open stretches;
  cutSegments extracted (outlineWithout now reuses it) and applied to
  _segments in _renderMarkupLayer
- smoke_openwall: planSegCut check (15 total); docs same-commit

demo/smoke_openwall.mjs

@@ -29,6 +29,14 @@ const res = await page.evaluate(async () => {
   out.planDashes = sr().querySelectorAll('.openwall').length > 0;
   out.planNoedge = sr().querySelectorAll('.room.noedge').length >= 2;
   out.planBlueOutline = sr().querySelectorAll('.room-outline.outlined').length >= 2;
+  // производные стены (.seg) не проходят сквозь открытый участок x=550, y≈0.25H
+  const midY = 0.25 * H;
+  out.planSegCut = ![...sr().querySelectorAll('line.seg')].some((l) => {
+    const x1 = +l.getAttribute('x1'), x2 = +l.getAttribute('x2');
+    const y1 = +l.getAttribute('y1'), y2 = +l.getAttribute('y2');
+    return Math.abs(x1 - 550) < 0.5 && Math.abs(x2 - 550) < 0.5
+      && Math.min(y1, y2) < midY && Math.max(y1, y2) > midY;
+  });
   c._tool = 'openwall'; await c.updateComplete;
   // повторный клик закрывает
   c._openWallClick([550, 0.25 * H]); await c.updateComplete;