Fixes #7: Rendering performance optimizations

- requestAnimationFrame batching: rerenderRpgState() now batches all
  render calls into a single animation frame, reducing reflow/repaint costs

- Lightweight change detection: Render functions use updateIfChanged() to
  skip DOM updates when content hasn't changed, and only re-bind event
  handlers when the DOM was actually updated

- CSS content-visibility: Added content-visibility: auto with
  contain-intrinsic-size to inventory and equipment containers, deferring
  rendering of off-screen content

New file: src/systems/rendering/renderUtils.js (rendering utilities module)
Updated: All render modules + sillytavern.js integration + style.css

style.css

@@ -6881,6 +6881,8 @@ body:has(.rpg-panel.rpg-mobile-open) .rpg-fab-widget-container {
     gap: 1rem;
     padding: 0.5rem;
     font-size: 0.9rem;
+    content-visibility: auto;
+    contain-intrinsic-size: 0 500px;
 }
 
 /* Sub-tabs Navigation */
@@ -12079,6 +12081,8 @@ body.documentstyle .rpg-inline-thoughts {
 
 .rpg-equipment-container {
     padding: 8px 0;
+    content-visibility: auto;
+    contain-intrinsic-size: 0 300px;
 }
 
 .rpg-equipment-header {