Rewrite Markdown links in merged PDFs

[0xtlt]

Summary

• detect relative .md links whose targets are included in a merged PDF
• rewrite matching PDF URI annotations as internal GoTo destinations to the target document's first page
• preserve unmatched Markdown links as URIs and document the new merge behavior

Testing

• cargo fmt --check
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked (55 unit tests and 26 CLI integration tests)
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --locked
• generated a two-page merged PDF and verified the link annotation is /S/GoTo to page 2 with no .md URI

Demo

No video: this changes PDF link behavior without changing visual layout.