feat(registry): add VFX and Captions block categories

Add 12 new registry blocks across two new categories:

**VFX Blocks (7):**
- vfx-text-cursor: dramatic text reveal with cursor glow and chromatic shadows
- vfx-liquid-background: organic liquid simulation with vertex displacement
- vfx-iphone-device: real GLTF iPhone + MacBook with live HTML screens
- vfx-magnetic: magnetic field particle visualization
- vfx-portal: dimension breach portal transition
- vfx-liquid-glass: Voronoi glass fracture with parallax reveal
- vfx-shatter: glass shatter with physics-driven fragments

**Caption Blocks (5):**
- captions-slam: bold uppercase scale-pop with back.out overshoot
- captions-karaoke: word-by-word color reveal on frosted glass pill
- captions-minimal: clean Netflix-subtitle style fade
- captions-bounce: playful elastic bounce with colorful word pills
- captions-cinematic: elegant slow fade with letter-spacing animation

All blocks include registry-item.json and are registered in registry.json.
VFX blocks use experimental stability, caption blocks use stable.

registry/blocks/vfx-liquid-background/registry-item.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://hyperframes.heygen.com/schema/registry-item.json",
+  "name": "vfx-liquid-background",
+  "type": "hyperframes:block",
+  "title": "Liquid Background",
+  "description": "Organic liquid simulation with vertex displacement on a subdivided plane. HTML content floats above rippling fluid surface with real-time wave dynamics.",
+  "stability": "experimental",
+  "dimensions": {
+    "width": 1920,
+    "height": 1080
+  },
+  "duration": 12,
+  "tags": ["vfx", "liquid", "webgl", "displacement", "background"],
+  "files": [
+    {
+      "path": "vfx-liquid-background.html",
+      "target": "compositions/vfx-liquid-background.html",
+      "type": "hyperframes:composition"
+    }
+  ]
+}