feat: editable labels for INPUT/OUTPUT/CLOCK gates

Double-click any INPUT, OUTPUT, or CLOCK gate to assign a custom label.
Labels are shown inside the gate and used in waveform viewer instead of
generic IN_0/OUT_0 names. Example circuits now ship with meaningful
labels (S, R, D, EN, Q, Q̅, CLK).

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

js/events.js

@@ -147,6 +147,20 @@ export function initEvents() {
         dragStartPos = null;
     });
 
+    // Double-click to rename INPUT/OUTPUT/CLOCK gates
+    canvas.addEventListener('dblclick', e => {
+        const world = screenToWorld(e.offsetX, e.offsetY);
+        const gate = findGateAt(world.x, world.y);
+        if (gate && (gate.type === 'INPUT' || gate.type === 'OUTPUT' || gate.type === 'CLOCK')) {
+            const current = gate.label || '';
+            const label = prompt(`Label for ${gate.type}#${gate.id}:`, current);
+            if (label !== null) {
+                gate.label = label.trim() || undefined;
+                console.log(`[label] ${gate.type}#${gate.id} → "${gate.label || ''}"`);
+            }
+        }
+    });
+
     canvas.addEventListener('contextmenu', e => {
         e.preventDefault();
         const world = screenToWorld(e.offsetX, e.offsetY);