feat: shift+drag to cut wires and create bus connectors

Hold Shift and drag across wires to create a BUS gate that groups
them together. The cut line shows a live preview with wire count.
BUS gates are pass-through (each input maps to its output) and
render as a thin cyan bar with ports on each side.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/state.js

@@ -42,5 +42,8 @@ export const state = {
     componentEditorActive: false,
     savedMainCircuit: null,  // { gates, connections, nextId } saved before entering editor
     componentEditorName: '',
-    editingComponentId: null  // ID of component being edited (null = new component)
+    editingComponentId: null,  // ID of component being edited (null = new component)
+
+    // Bus cutting (shift+drag)
+    busCutting: null  // { startX, startY, endX, endY } in world coords, or null
 };