[NIVEL 3] Issue #55 - TOC ScrollSpy con IntersectionObserver

Implementación completa de TOC sticky con scrollspy avanzado según Issue #55.

**Cambios en toc.css:**
- TOC sticky: position: sticky, top: 5.5rem, z-index: 10
- Border-left en links: 3px solid transparent (activo: #0d6efd)
- Scrollbar personalizado: width 6px, color #cbd5e0, hover #a0aec0
- Ajustados padding/margin para border-left

**Cambios en toc.js:**
- Reemplazado scroll handler por IntersectionObserver
- rootMargin: '-20% 0px -35% 0px' para detección óptima
- Tracking de headings visibles con Set
- Active link basado en primer heading visible
- Renombrado updateActiveLink → updateActiveLinkOnClick (evitar conflicto)
- Mantiene smooth scroll y reduce-motion support

**Características:**
✅ TOC sticky funcional con top: 5.5rem
✅ ScrollSpy con IntersectionObserver (rootMargin personalizado)
✅ Border-left 3px solid en active links
✅ Scrollbar width 6px, color #cbd5e0
✅ Smooth scroll con offset dinámico
✅ Performance optimizado (sin scroll events)
✅ Compatible todos los browsers modernos

Closes #55

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

wp-content/themes/apus-theme/assets/css/toc.css

@@ -19,7 +19,9 @@
     padding: 1.5rem;
     margin: 2rem 0;
     box-shadow: 0 2px 4px rgba(0, 0, 0, 0.05);
-    position: relative;
+    position: sticky;
+    top: 5.5rem;
+    z-index: 10;
 }
 
 .apus-toc-header {
@@ -170,11 +172,13 @@
 .apus-toc-link {
     color: #212529;
     text-decoration: none;
-    display: inline-block;
-    transition: color 0.2s ease, transform 0.2s ease;
+    display: block;
+    transition: color 0.2s ease, transform 0.2s ease, border-color 0.2s ease;
     line-height: 1.5;
     position: relative;
-    padding: 0.25rem 0;
+    padding: 0.25rem 0 0.25rem 1rem;
+    border-left: 3px solid transparent;
+    margin-left: -1rem;
 }
 
 .apus-toc-link:hover {
@@ -188,22 +192,11 @@
     border-radius: 2px;
 }
 
-/* Active link highlighting */
+/* Active link highlighting - Issue #55 */
 .apus-toc-link.active {
     color: #0d6efd;
     font-weight: 600;
-}
-
-.apus-toc-link.active::after {
-    content: '';
-    position: absolute;
-    left: -1rem;
-    top: 50%;
-    transform: translateY(-50%);
-    width: 4px;
-    height: 100%;
-    background-color: #0d6efd;
-    border-radius: 2px;
+    border-left-color: #0d6efd;
 }
 
 /* ========================================
@@ -348,16 +341,16 @@ h3[id] {
 }
 
 .apus-toc-list::-webkit-scrollbar-thumb {
-    background: #888;
+    background: #cbd5e0;
     border-radius: 3px;
 }
 
 .apus-toc-list::-webkit-scrollbar-thumb:hover {
-    background: #555;
+    background: #a0aec0;
 }
 
 /* Firefox */
 .apus-toc-list {
     scrollbar-width: thin;
-    scrollbar-color: #888 #f1f1f1;
+    scrollbar-color: #cbd5e0 #f1f1f1;
 }