feat: Added <amp-blurhash> component to render BlurHash placeholders for AMP pages

[sloppysaint]

Adds a new AMP component that renders BlurHash-encoded image placeholders using an HTML canvas. This is useful for displaying lightweight, visually coherent previews before the actual image loads, improving perceived performance and user experience.

Key features:

• Renders a valid BlurHash string using canvas.
• Custom element layout support (fixed, responsive, etc.).
• Lightweight: ~150 LOC, zero production dependencies.
• Fully unit-tested with coverage.
• Includes example page (examples/amp-blurhash.html) for easy testing.
• Validator rule added for runtime support.

This component is ideal for image-heavy AMP pages where placeholder polish matters.