Fix TOC page numbers

[jdillard]

Summary

Fix PDF TOC entries missing page numbers when building large multi-page singlehtml docs.

Problem

Fixes useblocks#120.

In singlehtml/simplepdf builds, section IDs collide after merging documents, so TOC target-counter links fail. Sphinx 8.2+ also strips docname-qualified refuri values via fix_refuris, leaving bare #introduction hrefs that do not match body anchors.

Approach

• Prefix section refid / ids with docname in SimplePdfBuilder.assemble_doctree() using /{docname}/#{id} (same approach as [singlehtml] add docname to section anchor to make them unique sphinx-doc/sphinx#13739).
• Align assemble_toc_secnumbers() / assemble_toc_fignumbers() and SimplepdfTranslator lookups.
• Override fix_refuris() so qualified sidebar TOC links are preserved.
• Override get_target_uri() to #/{docname}/ for document-level links.

Test plan

• New test: duplicate section titles across two source files → unique IDs in HTML
• New test: sidebar TOC section hrefs resolve to existing anchors
• pytest green in CI (WeasyPrint available)
• Manual: build demo project PDF and verify TOC page numbers

Related

• TOC Generation broken with newer Sphinx versions useblocks/sphinx-simplepdf#112 (Sphinx 8.2+ href format)
• [singlehtml] add docname to section anchor to make them unique sphinx-doc/sphinx#13739 (upstream; not required for this PR)