task(CAD-7): magnetic truss tool - drag placement with length options, snapTrussToDock end-to-end docking with rotation inheritance, opposite direction at start endpoints, truss rendering with belt lines

frontend/src/render/pixi/elementDrawers.ts

@@ -399,6 +399,42 @@ export function elementToPrimitives(el: CADElement, attrDefs?: Array<{ tag: stri
       return [];
     }
 
+    case 'truss': {
+      // Traverse (CAD-7): Balken als Rechteck + 2 Gurtlinien entlang
+      // der Rotation (wie ein Fachwerk-Profil)
+      const length = (props.length as number | undefined) ?? el.width;
+      const rotDeg = (props.rotation as number | undefined) ?? 0;
+      const rad = (rotDeg * Math.PI) / 180;
+      const dx = Math.cos(rad) * (length / 2);
+      const dy = Math.sin(rad) * (length / 2);
+      const nx = -Math.sin(rad) * 10; // Normalen-Offset (halbe Hoehe)
+      const ny = Math.cos(rad) * 10;
+      const c = { x: el.x, y: el.y };
+      return [
+        {
+          kind: 'rect',
+          x: c.x - dx, y: c.y - dy,
+          w: length, h: 20,
+          fill: fillOrNull(props.fill) ?? '#6b7280',
+          stroke: (props.stroke as string | undefined) ?? '#9aa0a6',
+          strokeWidth: 2,
+        },
+        // Gurtlinien oben/unten entlang der Traverse
+        {
+          kind: 'line',
+          from: { x: c.x - dx + nx, y: c.y - dy + ny },
+          to: { x: c.x + dx + nx, y: c.y + dy + ny },
+          stroke: '#4b5563', strokeWidth: 1,
+        },
+        {
+          kind: 'line',
+          from: { x: c.x - dx - nx, y: c.y - dy - ny },
+          to: { x: c.x + dx - nx, y: c.y + dy - ny },
+          stroke: '#4b5563', strokeWidth: 1,
+        },
+      ];
+    }
+
     case 'barrier': {
       // Absperrung (Task E7): Kette aus Kreis-Paaren (properties.circles).
       const circles = Array.isArray(props.circles)