Blurry placeholder performance: (Optionally) use sharp

[sharp](https://www.npmjs.com/package/sharp) should be a much faster image manipulation library. Would be nice if it could be the default, but otherwise making it optional would also be work.

The issue names no files or tests; start by locating the existing blurry-placeholder image-manipulation entry point and checking how dependencies are managed. Compare that path with sharp's supported API, then clarify whether default or optional use is intended; done requires a decided integration path and verification of the placeholder output and performance claim.

Written by the indexing model from the issue text.