feat(download): rename Raw tab to Download and add HTML/RTF/PDF page export

[tkuhn]

Closes #578.

The Raw tab is now the Download tab (?tab=raw kept as a legacy alias) and offers the page's content as downloadable documents next to the existing RDF links: HTML, RTF (opens in Word and LibreOffice, covering the Word/ODF ask without heavy dependencies), and PDF.

What's in here

• New export package: a format-neutral DocumentModel (sections, paragraphs, tables, lists) built by DocumentModelBuilder from the page's view displays, mirroring the QueryResult* rendering conventions (_label fallbacks, _noheader, multi-value columns, action-mapping/source columns dropped, header views as headings, query-form views skipped). IRIs link to the IRI itself so documents carry resolvable links.
• Writers: HtmlWriter (no dependency), hand-rolled RtfWriter (no dependency), and PdfWriter via the only new dependency, io.github.openhtmltopdf:openhtmltopdf-pdfbox (with a strip-and-retry fallback for ill-formed embedded paragraph HTML).
• DownloadDocPage at /download-doc (type, id, context, format=pdf|rtf|html); HTML serves inline, RTF/PDF as attachments. DownloadRdfLinks renamed to DownloadLinks with a "Document formats" section above the RDF sections.
• Shared view-display resolution: downloads now use the new public AbstractResourceWithProfile.fetchViewDisplaysSync(), reusing the page-rendering logic (ref-scoped query for spaces, preset-supplied views, own-classes matching). The previously duplicated copy in DownloadRdfPage had drifted and silently dropped class-targeted and preset views — this also fixes the existing RDF download. Query fetching moved to ViewDataFetcher; buildQueryRef gained ViewList's context/root_np auto-fill.
• PDF emoji: monochrome Noto Emoji (OFL, license bundled) registered as a fallback font so view-title emoji render as glyphs instead of #; fonts/** excluded from Maven resource filtering, which corrupts binaries.

Testing

• 953 tests green, including new unit tests for the writers (RTF escaping/surrogate pairs, XHTML strictness, PDF magic bytes + emoji-glyph regression via PDFBox text extraction), the builder's column conventions, and the page constants.
• Verified end-to-end on a local instance: tab rename + ?tab=raw alias, all three formats on space/user/part pages (PDF inspected visually and via pdftotext, RTF round-tripped through LibreOffice), export sections match the Content tab exactly, RDF links unchanged.

Notes

• Cold-cache exports share the RDF download's existing behavior (up to 30s query wait; sparse if the view-display cache is cold — warm in practice since the user just viewed the page).
• CJK text still renders as missing glyphs in PDF (base-14 fonts); a DejaVu fallback could be added the same way later.
• .docx/.odt writers can be added on the same DocumentModel if RTF proves insufficient.

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