refactor: modularize into ES6 modules

Split monolithic index.html into:
- js/constants.js - gate config, colors, dimensions
- js/state.js     - shared application state
- js/gates.js     - evaluation logic, port geometry
- js/renderer.js  - canvas drawing
- js/waveform.js  - GTKWave-style signal viewer
- js/simulation.js - clock tick engine
- js/events.js    - mouse, keyboard, UI handlers
- js/app.js       - entry point
- css/style.css   - all styles

js/constants.js

@@ -0,0 +1,26 @@
+// Gate dimensions and rendering constants
+export const GATE_W = 100;
+export const GATE_H = 60;
+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'
+};
+
+export const SIGNAL_COLORS = [
+    '#00e599', '#3388ff', '#ff6644', '#e5cc00',
+    '#cc44ff', '#ff44aa', '#ff8833', '#44ddff',
+    '#88ff44', '#ff4488', '#44ffaa', '#ffaa44'
+];
+
+export function gateInputCount(type) {
+    if (type === 'CLOCK' || type === 'INPUT') return 0;
+    if (type === 'NOT' || type === 'OUTPUT') return 1;
+    return 2;
+}
+
+export function gateOutputCount(type) {
+    return type === 'OUTPUT' ? 0 : 1;
+}