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/events.js

@@ -10,6 +10,7 @@ import { getLevel } from './levels.js';
 import { saveState, loadState, exportAsFile, importFromFile } from './saveLoad.js';
 import { enterComponentEditor, editComponentBlueprint, exitComponentEditor, updateComponentButtons, setResizeCallback } from './components.js';
 import { getExampleList, loadExample } from './examples.js';
+import { createBusFromCut } from './bus.js';
 
 const PAN_SPEED = 40;
 
@@ -31,6 +32,14 @@ export function initEvents() {
 
         // Convert to world coords for gate/port detection
         const world = screenToWorld(e.offsetX, e.offsetY);
+
+        // Update bus cut line endpoint
+        if (state.busCutting) {
+            state.busCutting.endX = world.x;
+            state.busCutting.endY = world.y;
+            return;
+        }
+
         state.hoveredPort = findPortAt(world.x, world.y);
         state.hoveredGate = state.hoveredPort ? state.hoveredPort.gate : findGateAt(world.x, world.y);
 
@@ -65,6 +74,19 @@ export function initEvents() {
         dragStartPos = { x: e.offsetX, y: e.offsetY };
         dragMoved = false;
 
+        // Shift+click on empty space → start bus cut
+        if (e.shiftKey && !state.placingGate) {
+            const port = findPortAt(world.x, world.y);
+            const gate = findGateAt(world.x, world.y);
+            if (!port && !gate) {
+                state.busCutting = {
+                    startX: world.x, startY: world.y,
+                    endX: world.x, endY: world.y
+                };
+                return;
+            }
+        }
+
         // Placing a new gate
         if (state.placingGate) {
             let w = GATE_W, h = GATE_H;
@@ -132,6 +154,13 @@ export function initEvents() {
     });
 
     canvas.addEventListener('mouseup', e => {
+        // Finish bus cut
+        if (state.busCutting) {
+            createBusFromCut();
+            state.busCutting = null;
+            return;
+        }
+
         // Toggle INPUT/CLOCK only on click (no drag movement)
         if (state.dragging && !dragMoved) {
             const gate = state.dragging;