[FIX] Cross-page links resolve in the merged PDF

[eleanorfrajka]

Summary

Cross-page links in the merged mooring PDF (the Summary/Stack/Grid/serial header pills and the instrument links) were dead: combine_mooring_pdf rendered each source HTML file in its own render() call and page-concatenated them, and WeasyPrint only resolves internal links within a single render(). This rebuilds the combiner to assemble one HTML document and render it once, so the links resolve, and makes tall tables paginate sensibly.

What's changed

• Single-document combine. combine_mooring_pdf now reads the ordered report files, assembles them into one HTML document via a pure, tested string transform (_build_combined_html + helpers in reports/_pdf.py), and renders that once.

• Per-page id namespacing. Ids are reused across pages (top, qc, ts, vars, …), so the transform rewrites id="x" → id="{slug}__x" and same-page href="#x" → href="#{slug}__x" (slugs summary/stack/grid/instr-{serial}) before concatenation; otherwise every #top would resolve to the first page.

• Inter-page link rewriting. Links to sibling report files ({mooring}_stack_report.html, the instrument/…#start and ../ forms) become in-document anchors targeting each page's masthead #{slug}__top (every page carries id="top" from base.html).

• Regex, not an HTML parser. Safe because the report CSS has no id selectors and figures are base64 PNG, not inline SVG — a SAFETY CONDITION comment in _pdf.py records that SVG would also require rewriting url(#…)/xlink:href.

• Table pagination. _PRINT_CSS now lets tables break at row boundaries (tr { break-inside: avoid }), repeats the header row on continuation pages (thead { display: table-header-group }), and keeps headings with their content (h2,h3,h4 { break-after: avoid }) — instead of forcing a tall table whole onto the next sheet.

• Tests. test_pdf_report.py: slug derivation, the pure transform (namespacing, inter-page + fragment rewrite, style dedup, .pdf-page wrapping, a completeness tripwire scanning broader than the transform), and a WeasyPrint-gated end-to-end test asserting the link resolves to an internal /Dest with no dead .html. Verified on the real dune2 fixture.

Notes

• No public API or CLI change: combine_mooring_pdf's signature and output path are unchanged. The PDF's internal layout changes (links now internal; tables paginate at rows), but nothing parses the PDF — no breaking change for callers.
• Render/end-to-end tests are gated on importorskip("weasyprint") (the pdf extra); they skip without it, as the existing PDF tests already do.
• Known limitation (deferred): on the "Global attributes" subhead the heading can still strand at a page bottom — WeasyPrint 69 does not honour break-after: avoid there and it is not reproducible in a minimal case. Logged for follow-up (candidate fix: emit the heading as a table <caption>).