feat(docx): export a link as a link, and an anchor as a bookmark

[DemchaAV]

Why

Every link the document carried was dropped. A reader opened an export and found the text
of a link with nothing behind it, and a reference to another section that went nowhere. For
a document handed over to be edited rather than read, that is most of the reason for handing
it over at all — a PDF at least keeps its links.

What changed

A linkTarget becomes a w:hyperlink — a relationship carrying the address for an
external one, w:anchor for one of the document's own anchors. An external link goes
through POI's createHyperlinkRun, which makes the relationship the part needs; POI has no
helper for an anchor, so that one is built here.

A run's own link wins over the paragraph's. A sentence with one linked phrase in it is
the ordinary case: the paragraph's link is the fallback for the rest of the sentence rather
than something the phrase overrides away.

An anchor(...) becomes a w:bookmarkStart / w:bookmarkEnd pair wrapping the
paragraph's text. Opened and closed in two calls, because both elements append to the end
of the paragraph — doing it in one leaves a bookmark around nothing and a reader landing
above the text instead of on it.

Names are cleaned once and remembered (DocxBookmarkNames). Word takes far less of a
name than a document gives: letters, digits and underscores, starting with a letter, forty
characters. A name it dislikes is not a broken link but a file it refuses to open. A link
and the bookmark it points at are written at different times by different parts of the
export, so both go through one mapping — Terms & conditions becomes Terms_conditions on
both sides — and two anchors that clean to one name stay two bookmarks rather than
collapsing onto each other. Forward references need nothing extra: neither side resolves
anything at write time.

The outline is deliberately not written. bookmarkOptions builds a PDF outline tree;
Word's Navigation Pane comes from heading styles, and promoting an anchored paragraph to a
heading would restyle the document. The capability matrix and the DOCX recipe say so rather
than leaving it to be discovered.

Verification

./mvnw -B -ntp clean verify over the seven-module gate → BUILD SUCCESS, 1784 tests.
./mvnw -B -ntp test -f examples/pom.xml → 93 tests, BUILD SUCCESS; no committed preview
drifted.

render-docx goes from 182 to 190 tests. DocxLinkTest covers a paragraph link and the
relationship behind it, one linked phrase among plain runs, an internal link matching the
bookmark its anchor became, a forward reference, the bookmark wrapping the text rather than
sitting before it, two anchors that clean to one name staying apart, an anchor starting with
a digit, and a plain paragraph carrying neither.

Checked by opening the export in Word 16.0 and reading its own PDF back: two live links,
one to the address and one jumping within the document. Behaviour, not the presence of the
XML — the same standard the font work was held to.

Lane: canonical — document.backend.semantic.docx only; no engine, layout or pagination
internals are touched.