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

src/logic.ts

@@ -912,14 +912,13 @@ export function openZoneOf(roomId: string, rooms: { id?: string; open_to?: strin
 }
 
 /**
- * Room outline pieces with the given collinear stretches removed — used to
- * draw a TRUE dashed open boundary (the solid stroke must not run beneath).
- * Returns segments [x1,y1,x2,y2].
+ * Segments with the given collinear stretches removed — the workhorse behind
+ * TRUE dashed open boundaries (derived walls and room outlines alike).
  */
-export function outlineWithout(poly: number[][], cuts: number[][], eps = 1e-6): number[][] {
+export function cutSegments(segs: number[][], cuts: number[][], eps = 1e-6): number[][] {
   const out: number[][] = [];
-  for (let i = 0; i < poly.length; i++) {
-    const p1 = poly[i], p2 = poly[(i + 1) % poly.length];
+  for (const seg of segs) {
+    const p1 = [seg[0], seg[1]], p2 = [seg[2], seg[3]];
     const dx = p2[0] - p1[0], dy = p2[1] - p1[1];
     const len = Math.hypot(dx, dy);
     if (len < eps) continue;
@@ -952,6 +951,16 @@ export function outlineWithout(poly: number[][], cuts: number[][], eps = 1e-6):
   return out;
 }
 
+/** Room outline pieces with the given collinear stretches removed. */
+export function outlineWithout(poly: number[][], cuts: number[][], eps = 1e-6): number[][] {
+  const edges: number[][] = [];
+  for (let i = 0; i < poly.length; i++) {
+    const p1 = poly[i], p2 = poly[(i + 1) % poly.length];
+    edges.push([p1[0], p1[1], p2[0], p2[1]]);
+  }
+  return cutSegments(edges, cuts, eps);
+}
+
 /** Distance from a point to a segment [x1,y1,x2,y2]. */
 export function distToSegment(p: number[], s: number[]): number {
   const dx = s[2] - s[0], dy = s[3] - s[1];