feat: add OpenGraph image generation

[Princesseuh]

What does this change?

• Adds ctx.assets.add_opengraph_image(svg), a built-in way to generate OpenGraph images from an SVG string. The SVG is rendered to a PNG at build time using resvg.
• Returns an OpenGraphImage with url(), width(), height(), and render() — the latter emits the <meta property="og:image"> tags. As with images, referencing the result in the page is opt-in; the image is generated whether or not render() is called.
• Locked behind the new og_image crate feature, enabled by default. resvg is only pulled in when the feature is on.

let og = ctx.assets.add_opengraph_image(
    r##"<svg xmlns="http://www.w3.org/2000/svg" width="1200" height="630">
          <rect width="100%" height="100%" fill="#1a1a1a"/>
          <text x="60" y="330" fill="white" font-size="72">Hello, world!</text>
        </svg>"##,
)?;

html! { head { (og.render()) } }

The generated PNG is materialized on disk and registered as a regular image asset, so it flows through the existing asset copy, hashing, caching, and stale-cleanup pipeline with no changes to build.rs.

Note: resvg renders SVG, not arbitrary HTML (HTML would need a browser/layout engine). SVG is easy to produce from maud/format!, so the API takes an SVG string.

How is it tested?

• Unit tests in opengraph.rs: asset creation and PNG output, emitted meta tags, content-hash dedup (same SVG → same URL/file), and error on invalid SVG.
• Verified end-to-end with a real coronate build: the PNG is written to the output dir (valid 1200×630), the og:image URL in the rendered HTML matches the file on disk, and it survives the bundling/substitution pass.
• Full existing suite passes with --all-features, --no-default-features, and --features maud; clippy is clean in each configuration.

How is it documented?

• Rustdoc on the module, OpenGraphImage, and add_opengraph_image, including a usage example (doctest).
• A Sampo changeset (cargo/maudit: minor).
• The kitchen-sink example is updated to generate and reference an OpenGraph image.

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