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/state.js

@@ -0,0 +1,32 @@
+// Shared application state — single source of truth
+export const state = {
+    // Circuit
+    gates: [],
+    connections: [],
+    nextId: 1,
+
+    // Interaction
+    placingGate: null,
+    dragging: null,
+    dragOffset: { x: 0, y: 0 },
+    connecting: null,
+    hoveredGate: null,
+    hoveredPort: null,
+    mouseX: 0,
+    mouseY: 0,
+
+    // Waveform
+    waveformVisible: false,
+    waveformHeight: 220,
+    recording: true,
+    waveData: {},       // { gateId: [{ t, value }] }
+    timeStep: 0,
+    waveZoom: 20,       // pixels per time step
+    waveScroll: 0,
+    resizingWave: false,
+
+    // Simulation
+    simRunning: false,
+    simInterval: null,
+    simSpeed: 500        // ms per tick
+};