feat(rebar): apply 4-bar symmetric vertical stacking offset (±1.5 db, ±0.5 db) to P6 crossties to resolve centre intersection interference

src/view/rebar3d.ts

@@ -214,20 +214,24 @@ export function buildRebar3D(m: PierModel): RebarScene {
     for (const z of col.p5) ring('P5', c.k, sc, base + mm(z), rt, NOMINAL_DIAMETER.H19);
     for (const z of col.p4) ring('P4', c.k, sc, base + mm(z), rt, NOMINAL_DIAMETER.H19);
     const hookP6 = mm(Math.round(hookLength('tie90', NOMINAL_DIAMETER.H16))); // 135 mm
+    const dP6 = NOMINAL_DIAMETER.H16; // 15.9 mm
+    const p6Angles = [0, 46.96, 90, 133.04];
     for (const z of col.p6Levels) {
-      for (const deg of [0, 46.96, 90, 133.04]) {
+      p6Angles.forEach((deg, i) => {
         const a = (deg * Math.PI) / 180;
         const xA = sc - rt * Math.cos(a);
         const zA = -rt * Math.sin(a);
         const xB = sc + rt * Math.cos(a);
         const zB = rt * Math.sin(a);
-        const yBar = base + mm(z);
+        // Vertical stacking offset: ±1.5 db, ±0.5 db to avoid interference at the column centre (Q-C15)
+        const zOffset = (i - 1.5) * dP6;
+        const yBar = base + mm(z + zOffset);
         // Main crosstie across the diameter
-        addBar('P6', c.k, [xA, yBar, zA], [xB, yBar, zB], NOMINAL_DIAMETER.H16, COLOR.tie);
+        addBar('P6', c.k, [xA, yBar, zA], [xB, yBar, zB], dP6, COLOR.tie);
         // 90° hooks at both ends turning downward (135 mm, wrapping the tie)
-        addBar('P6', c.k, [xA, yBar, zA], [xA, yBar - hookP6, zA], NOMINAL_DIAMETER.H16, COLOR.tie);
-        addBar('P6', c.k, [xB, yBar, zB], [xB, yBar - hookP6, zB], NOMINAL_DIAMETER.H16, COLOR.tie);
-      }
+        addBar('P6', c.k, [xA, yBar, zA], [xA, yBar - hookP6, zA], dP6, COLOR.tie);
+        addBar('P6', c.k, [xB, yBar, zB], [xB, yBar - hookP6, zB], dP6, COLOR.tie);
+      });
     }
   }