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/waveform.js

@@ -12,6 +12,7 @@ export function getTrackedGates() {
 }
 
 export function getGateLabel(gate) {
+    if (gate.label) return gate.label;
     const sameType = state.gates.filter(g => g.type === gate.type);
     const idx = sameType.indexOf(gate);
     if (gate.type === 'CLOCK') return `CLK_${idx}`;