StylesheetLinker

Injects <link rel="stylesheet" .../> tags into every XHTML resource for each href in Book.stylesheets. Idempotent — re-running it (or running it on a book that already has links) is a no-op.

Path resolution: stylesheet hrefs are made relative to each page's own href, so a stylesheet at styles/book.css becomes styles/book.css from chapter-1.xhtml and ../styles/book.css from text/chapter-1.xhtml.

Wired into EpubWriter's default pipeline. Users who don't want this can pass their own BookProcessor to the EpubWriter constructor.

When Book.stylesheets is empty this processor returns immediately and lazy-loaded resources stay lazy. Otherwise every XHTML page is read and rewritten, materializing any lazy XHTML for the rest of the write.