Trail casing: dark halo + light core

The accent line vanished on same-hue fills. Blend modes (difference/
exclusion) all keep a blind luminance where the stroke disappears and
composite expensively on old kiosk WebViews, so the trail now uses the
cartographers' trick instead: a neutral dark halo under a light core —
one of the two always contrasts with whatever is underneath. Verified
over glow fill (dark rooms + light pools) in one screenshot.

src/houseplan-card.ts

@@ -4541,7 +4541,10 @@ class HouseplanCard extends LitElement {
             const [cx, cy] = applyAffine(matrix, x, y);
             return cx.toFixed(1) + ',' + cy.toFixed(1);
           }).join(' ');
-          trails.push(svg`<polyline points="${ptsStr}"></polyline>`);
+          // cartography casing: a dark halo under a light core. Neutral and
+          // visible over ANY room fill — blend modes all have a blind
+          // luminance where the line vanishes (owner request 2026-07-31).
+          trails.push(svg`<polyline class="case" points="${ptsStr}"></polyline><polyline class="core" points="${ptsStr}"></polyline>`);
         }
       }
       if (!moving || !tele.pos) continue;