fix: use requestAnimationFrame + real timestamps for clock simulation

Replace setInterval with requestAnimationFrame loop that tracks elapsed
time via performance.now(). Clock ticks now fire based on real time
rather than assuming perfect interval spacing, so user interactions
(toggling inputs, dragging gates) no longer cause the clock to stutter
or pause.

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

js/simulation.js

@@ -1,9 +1,18 @@
-// Clock simulation engine
+// Clock simulation engine — uses real timestamps for consistent timing
 import { state } from './state.js';
 import { evaluateAll } from './gates.js';
 import { forceRecordSample } from './waveform.js';
 
-export function simTick() {
+let lastTickTime = 0;
+let animFrameId = null;
+
+function simLoop(now) {
+    if (!state.simRunning) return;
+
+    // Fire ticks that are due based on real elapsed time
+    while (now - lastTickTime >= state.simSpeed) {
+        lastTickTime += state.simSpeed;
+
         // Toggle all CLOCK gates
         state.gates.forEach(g => {
             if (g.type === 'CLOCK') {
@@ -11,10 +20,12 @@ export function simTick() {
             }
         });
         evaluateAll();
-    // Force record even if evaluateAll didn't detect change
         if (state.recording && state.waveformVisible) {
             forceRecordSample();
         }
+    }
+
+    animFrameId = requestAnimationFrame(simLoop);
 }
 
 export function startSim() {
@@ -29,18 +40,18 @@ export function startSim() {
         state.waveformVisible = true;
         document.getElementById('waveform-panel').classList.add('visible');
         document.getElementById('sim-btn').classList.add('active');
-        // Trigger resize via event so renderer picks it up
         window.dispatchEvent(new Event('resize'));
     }
 
-    state.simInterval = setInterval(simTick, state.simSpeed);
+    lastTickTime = performance.now();
+    animFrameId = requestAnimationFrame(simLoop);
     updateSimUI();
 }
 
 export function stopSim() {
     state.simRunning = false;
-    if (state.simInterval) clearInterval(state.simInterval);
-    state.simInterval = null;
+    if (animFrameId) cancelAnimationFrame(animFrameId);
+    animFrameId = null;
     updateSimUI();
 }
 
@@ -57,10 +68,8 @@ export function updateSimUI() {
 }
 
 export function adjustSpeed(delta) {
+    const wasRunning = state.simRunning;
     state.simSpeed = Math.max(50, Math.min(2000, state.simSpeed + delta));
-    if (state.simRunning) {
-        clearInterval(state.simInterval);
-        state.simInterval = setInterval(simTick, state.simSpeed);
-    }
+    // No need to restart — the loop reads simSpeed dynamically
     updateSimUI();
 }