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/data-chart/data-chart.html

@@ -122,7 +122,7 @@
 
         [data-composition-id="data-chart"] .subtitle {
           font-size: 16px;
-          color: #d4d4d4;
+          color: #666;
           margin: 8px 0 16px 0;
           clip-path: inset(0 100% 0 0);
           white-space: nowrap;
@@ -205,12 +205,8 @@
       </style>
 
       <script>
-        (function initChart() {
+        (function () {
           const svg = document.querySelector('[data-composition-id="data-chart"] .chart-svg');
-          if (!svg) {
-            setTimeout(initChart, 50);
-            return;
-          }
           const tl = gsap.timeline({ paused: true });
 
           // Data