feat(registry): seed transition blocks — 14 shader + 14 CSS showcase (#270)

## What

Add 28 transition blocks from the Hyperframe Template Structure catalog, bringing the registry to 53 total items.

### Shader transitions (14 blocks, WebGL, 4s each)
`domain-warp-dissolve`, `ridged-burn`, `whip-pan`, `sdf-iris`, `ripple-waves`, `gravitational-lens`, `cinematic-zoom`, `chromatic-radial-split`, `glitch`, `swirl-vortex`, `thermal-distortion`, `flash-through-white`, `cross-warp-morph`, `light-leak`

### CSS transition showcases (14 blocks, various durations)
`transitions-3d`, `transitions-blur`, `transitions-cover`, `transitions-destruction`, `transitions-dissolve`, `transitions-distortion`, `transitions-grid`, `transitions-light`, `transitions-mechanical`, `transitions-other`, `transitions-push`, `transitions-radial`, `transitions-scale`, `transitions-shader`

## Why

Phase D content accumulation. Transitions are the most-requested category for the catalog.

## How

- Shader transitions extracted from `shader-showcase.zip`, each a standalone HTML with WebGL shaders
- CSS transitions extracted from `showcase-bundle.zip`, each a standalone showcase page
- All tagged with `transition` + `shader` or `showcase` for catalog grouping
- Preview thumbnails generated for all 28 blocks
- Catalog pages + index regenerated

## Test plan

- [x] All 28 blocks produce preview thumbnails
- [x] `registry-item.json` validates for all blocks
- [x] Catalog pages generated (45 total items in catalog-index.json)
- [x] `oxfmt --check` passes

registry/blocks/flash-through-white/registry-item.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://hyperframes.heygen.com/schema/registry-item.json",
+  "name": "flash-through-white",
+  "type": "hyperframes:block",
+  "title": "Flash Through White",
+  "description": "Shader transition with white flash crossfade",
+  "tags": ["transition", "shader"],
+  "dimensions": {
+    "width": 1920,
+    "height": 1080
+  },
+  "duration": 4,
+  "files": [
+    {
+      "path": "flash-through-white.html",
+      "target": "compositions/flash-through-white.html",
+      "type": "hyperframes:composition"
+    }
+  ]
+}