feat: support ThumbHash as placeholder - #12

Summary

Adds opt-in ThumbHash placeholders alongside the existing inline image and BlurHash previews.

imagePlugin({
  local: {
    sizes: [480, 800, 1200],
    placeholder: { type: "thumbhash" },
  },
});

thumbhash is an optional peer dependency and is only imported when this preview type is enabled.

Why

ThumbHash is a useful alternative to BlurHash because it:

• has a compact binary representation;
• supports transparency;
• includes an approximate aspect ratio;
• does not require component-count configuration.

This implementation keeps the hash as a Uint8Array, matching the upstream thumbhash API instead of introducing an internal base64 representation.

Implementation

• Add { type: "thumbhash" } to the Vite plugin placeholder options.
• Encode local images with rgbaToThumbHash from an EXIF-corrected, max-100px RGBA sample.
• Preserve alpha in the server-side average placeholder color.
• Decode in the browser with thumbHashToDataURL.
• Keep SolidImage independent of thumbhash; generated modules inject the decoder only when needed.
• Support ThumbHash placeholders from remote.transformURL.
• Serialize cached/remote hash bytes as arrays and restore them to Uint8Array in generated modules.
• Keep the existing BlurHash API unchanged; string hashes remain BlurHash, binary hashes are ThumbHash.
• Add a ThumbHash example, docs, tests, and a minor changeset.

Testing

Validated with:

• frozen pnpm install;
• package build;
• all example builds;
• Node tests;
• Playwright Chromium browser tests.

Coverage includes local and remote placeholders, EXIF orientation, the 100px encoding limit, alpha-aware colors, Uint8Array restoration, decoder injection, browser decoding, and preview cleanup after image load.