Initial commit: cell holder generator + busbar builder

- 3D printable cell holder with STEP export (grid/honeycomb/vertical layouts)
- Configurable BMS holes (half circles, full circles, edge tabs)
- Busbar builder: click cells to define busbars, auto-routed with bend fallback
- Compound STEP export bundles holder + busbars
- OpenCascade.js (wasm) vendored for 3D modeling

src/busbar-model.js

@@ -0,0 +1,57 @@
+import { ocRef } from './oc.js';
+
+export function build3DBusbar(geometry, positions, padRadius, zBase, thickness) {
+    const oc = ocRef.instance;
+    if (!oc || geometry.padIndices.length === 0) return null;
+
+    const shapes = [];
+
+    for (const i of geometry.padIndices) {
+        if (!positions[i]) continue;
+        const [x, y] = positions[i];
+        const ax = new oc.gp_Ax2(new oc.gp_Pnt(x, y, zBase), oc.gp.prototype.DZ());
+        shapes.push(new oc.BRepPrimAPI_MakeCylinder(ax, padRadius, thickness).Shape());
+    }
+
+    for (const edge of geometry.edges) {
+        const pts = [positions[edge.from], ...edge.waypoints, positions[edge.to]];
+
+        for (let k = 1; k < pts.length - 1; k++) {
+            const [wx, wy] = pts[k];
+            const ax = new oc.gp_Ax2(new oc.gp_Pnt(wx, wy, zBase), oc.gp.prototype.DZ());
+            shapes.push(new oc.BRepPrimAPI_MakeCylinder(ax, padRadius, thickness).Shape());
+        }
+
+        for (let k = 0; k < pts.length - 1; k++) {
+            const [x1, y1] = pts[k];
+            const [x2, y2] = pts[k + 1];
+            const dx = x2 - x1, dy = y2 - y1;
+            const len = Math.hypot(dx, dy);
+            if (len < 1e-6) continue;
+            const angle = Math.atan2(dy, dx);
+
+            const box = new oc.BRepPrimAPI_MakeBox(len, 2 * padRadius, thickness).Shape();
+
+            const trans1 = new oc.gp_Trsf();
+            trans1.SetTranslation(new oc.gp_Vec(0, -padRadius, 0));
+            let shape = new oc.BRepBuilderAPI_Transform(box, trans1, false).Shape();
+
+            const rot = new oc.gp_Trsf();
+            rot.SetRotation(new oc.gp_Ax1(new oc.gp_Pnt(0, 0, 0), oc.gp.prototype.DZ()), angle);
+            shape = new oc.BRepBuilderAPI_Transform(shape, rot, false).Shape();
+
+            const trans2 = new oc.gp_Trsf();
+            trans2.SetTranslation(new oc.gp_Vec(x1, y1, zBase));
+            shape = new oc.BRepBuilderAPI_Transform(shape, trans2, false).Shape();
+
+            shapes.push(shape);
+        }
+    }
+
+    if (shapes.length === 0) return null;
+    let combined = shapes[0];
+    for (let i = 1; i < shapes.length; i++) {
+        combined = new oc.BRepAlgoAPI_Fuse(combined, shapes[i]).Shape();
+    }
+    return combined;
+}