feat(registry): seed components — grain-overlay, shimmer-sweep, grid-pixelate-wipe (#260)

## What

Three reusable effect components for the registry, each with a snippet HTML and companion `demo.html`:

| Component | Description |
|-----------|-------------|
| `grain-overlay` | Animated film grain texture overlay (CSS keyframes, extracted from warm-grain example) |
| `shimmer-sweep` | CSS gradient light sweep across text/elements, driven by GSAP custom property animation |
| `grid-pixelate-wipe` | Grid-based dissolve transition — screen breaks into 16×9 squares that scale in/out with stagger |

Establishes the `demo.html` convention in `CONTRIBUTING.md`.

## Why

Phase B of the catalog plan — seed the first components in the registry. Components are effect snippets that get merged into existing compositions (vs. blocks which are standalone sub-compositions).

## How

- **grain-overlay**: Extracted the grain texture pattern from the warm-grain example. Uses a 200% oversized tiled texture with `steps(1)` keyframe animation for the random-noise effect.
- **shimmer-sweep**: Original implementation using CSS custom properties (`--shimmer-pos`) animated by GSAP. The gradient mask uses `mix-blend-mode: overlay` for a natural light sweep. Auto-injects `.shimmer-mask` elements into `.shimmer-sweep-target` wrappers.
- **grid-pixelate-wipe**: Creates a 16×9 CSS Grid of cells, animated with GSAP stagger. Users drive `.grid-cell` `scale` directly in their timeline.

Simplify review addressed: scoped `.grain-texture` under `#grain-overlay`, scoped `.grid-cell` under `#grid-pixelate-overlay`, removed `window.gridPixelateIn/Out` globals in favor of direct GSAP patterns.

Each component ships a `demo.html` — a standalone composition that previews the effect and doubles as a fixture for the CI preview pipeline (PR 8).

## Test plan

- [x] `hyperframes add grain-overlay` installs to `compositions/components/grain-overlay.html`
- [x] `hyperframes add shimmer-sweep` installs to `compositions/components/shimmer-sweep.html`
- [x] `hyperframes add grid-pixelate-wipe` installs to `compositions/components/grid-pixelate-wipe.html`
- [x] All three return correct `--json` output with snippet and type info
- [x] `registry-item.json` files validate against the JSON Schema
- [x] `demo.html` files are self-contained with correct `data-composition-id` and `window.__timelines` registration
- [x] `oxfmt --check` and `oxlint` pass on all files
- [x] `CONTRIBUTING.md` documents the `demo.html` convention and registry item checklist

registry/components/grid-pixelate-wipe/registry-item.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "https://hyperframes.heygen.com/schema/registry-item.json",
+  "name": "grid-pixelate-wipe",
+  "type": "hyperframes:component",
+  "title": "Grid Pixelate Wipe",
+  "description": "Transition effect where the screen dissolves into a grid of squares that fade out with staggered timing — use between scenes",
+  "tags": ["transition", "wipe", "grid", "pixelate"],
+  "files": [
+    {
+      "path": "grid-pixelate-wipe.html",
+      "target": "compositions/components/grid-pixelate-wipe.html",
+      "type": "hyperframes:snippet"
+    }
+  ]
+}