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/renderer.js

@@ -63,8 +63,18 @@ function drawGate(gate) {
     ctx.textBaseline = 'middle';
 
     const isIOType = gate.type === 'INPUT' || gate.type === 'OUTPUT' || gate.type === 'CLOCK';
+
+    // Show custom label above the gate if it has one
+    if (gate.label && isIOType) {
+        ctx.font = 'bold 10px "Segoe UI", system-ui, sans-serif';
+        ctx.fillStyle = '#888';
+        ctx.fillText(gate.label, gate.x + GATE_W / 2, gate.y - 8);
+    }
+
+    ctx.font = `bold 14px "Segoe UI", system-ui, sans-serif`;
+    ctx.fillStyle = isActive ? '#fff' : color;
     ctx.fillText(
-        gate.type === 'CLOCK' ? '⏱ CLK' : gate.type,
+        gate.label && isIOType ? gate.label : (gate.type === 'CLOCK' ? '⏱ CLK' : gate.type),
         gate.x + GATE_W / 2,
         gate.y + GATE_H / 2 - (isIOType ? 8 : 0)
     );