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

.github/workflows/catalog-previews.yml

@@ -18,6 +18,8 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
       - uses: oven-sh/setup-bun@v2
 
@@ -38,11 +40,12 @@ jobs:
       - uses: FedericoCarboni/setup-ffmpeg@v3
 
       - name: Render changed block/component previews
-        continue-on-error: true
         run: |
           # Find which blocks/components changed in this PR
-          CHANGED_ITEMS=$(git diff --name-only origin/main...HEAD -- registry/blocks/ registry/components/ \
-            | grep -oP '(?<=registry/(blocks|components)/)[^/]+' \
+          BASE_SHA=${{ github.event.pull_request.base.sha }}
+          CHANGED_ITEMS=$(git diff --name-only "$BASE_SHA"...HEAD -- registry/blocks/ registry/components/ \
+            | grep -E '^registry/(blocks|components)/' \
+            | sed 's|^registry/[^/]*/\([^/]*\)/.*|\1|' \
             | sort -u)
 
           if [ -z "$CHANGED_ITEMS" ]; then
@@ -55,7 +58,7 @@ jobs:
 
           for item in $CHANGED_ITEMS; do
             echo "Rendering preview for: $item"
-            if ! npx tsx scripts/generate-catalog-previews.ts --only "$item" --skip-video; then
+            if ! timeout 120 npx tsx scripts/generate-catalog-previews.ts --only "$item" --skip-video; then
               echo "::warning::Failed to render preview for $item"
               FAILED=$((FAILED + 1))
             fi
@@ -73,4 +76,4 @@ jobs:
           name: catalog-previews
           path: docs/images/catalog/
           if-no-files-found: ignore
-          retention-days: 30
+          retention-days: 7