Add debug logs to EventBus

Events now logs to console when _debug is true:
- [Events.on] event (N handlers) - green
- [Events.off] event - orange
- [Events.emit] event data → N handlers - blue

Toggle in browser console:
  Events._debug = false  // disable
  Events._debug = true   // enable (default)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

deck-frontend/deck-v4.6.html

@@ -808,12 +808,21 @@ const State = {
 // =============================================================================
 const Events = {
   _handlers: {},
-  on(event, fn) { (this._handlers[event] ||= []).push(fn); },
+  _debug: true,  // Toggle: Events._debug = false to disable
+  on(event, fn) {
+    (this._handlers[event] ||= []).push(fn);
+    if (this._debug) console.log(`%c[Events.on] ${event}`, 'color: #4CAF50', `(${this._handlers[event].length} handlers)`);
+  },
   off(event, fn) {
     if (!fn) delete this._handlers[event];
     else this._handlers[event] = (this._handlers[event] || []).filter(h => h !== fn);
+    if (this._debug) console.log(`%c[Events.off] ${event}`, 'color: #FF9800');
   },
-  emit(event, data) { (this._handlers[event] || []).forEach(fn => fn(data)); }
+  emit(event, data) {
+    const handlers = this._handlers[event] || [];
+    if (this._debug) console.log(`%c[Events.emit] ${event}`, 'color: #7c8aff; font-weight: bold', data !== undefined ? data : '', `→ ${handlers.length} handlers`);
+    handlers.forEach(fn => fn(data));
+  }
 };
 
 // =============================================================================