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/constants.js

@@ -7,7 +7,8 @@ export const PORT_R = 7;
 export const GATE_COLORS = {
     AND: '#00e599', OR: '#3388ff', NOT: '#ff6644',
     NAND: '#e5cc00', NOR: '#cc44ff', XOR: '#ff44aa',
-    INPUT: '#3388ff', CLOCK: '#ff44aa', OUTPUT: '#ff8833'
+    INPUT: '#3388ff', CLOCK: '#ff44aa', OUTPUT: '#ff8833',
+    BUS: '#44ddff'
 };
 
 export const SIGNAL_COLORS = [