fix(docx): give a table the space it holds above and below itself - #712

Why

Word has no space above a table and none below one, so a table's own margin and padding
were dropped on the way out. A RowNode is exported as a one-row table, so a row's padding
went the same way.

Measured on the probe corpus through LibreOffice against the reference PDF, that was three
of the four gaps still missing from page 1 — a row's 14pt lost at each edge, and the 6pt a
billing table holds above itself:

123.2 → 108.1  −14.0  Scope    ← the row's top padding
221.8 → 192.7  −14.0  Notice   ← the row's bottom padding
317.0 → 282.2   −5.7  Item     ← the table's top margin

They looked like two defects (a row's padding, a header row) and were one.

What changed

A table-shaped node's own box goes through the same gap as everything else.
writeTableWithItsOwnSpacing owes its top edge before the table is written — the flush that
already runs before a table pays it onto the paragraph above — and owes its bottom edge
after, which the next paragraph writes above itself. So a table between two paragraphs now
reads as two paragraphs with a gap between them, and nothing needs an element of its own.

The one edge Word cannot hold is still dropped. A table with no paragraph above it loses
its top edge rather than getting an empty paragraph to carry it, which would be a line the
document never asked for. A test pins that, and pins that the bottom edge still arrives.

Verification

./mvnw -B -ntp clean verify over the eight-module gate → BUILD SUCCESS (core 815,
render-pdf 337, render-docx 225, testing 5, render-pptx 140, templates 127, qa 1784).
./mvnw -B -ntp test -f examples/pom.xml → 93 tests, BUILD SUCCESS; no committed preview
drifted.

Measured again on the same corpus and the same editor, page 1 end to end:

 57.1 →  57.5  +0.4  Quarterly service report
123.2 → 122.1  −1.1  Scope
221.8 → 220.7  −1.1  Notice
294.2 → 293.1  −1.1  Billing
317.0 → 316.2  −0.8  Item
400.8 → 399.6  −1.2  Additional storage

Every block is within 1.2pt of the page, against 87pt of accumulated drift three changes
ago. The residue is the 1.1pt the first paragraph starts with and never grows.

render-docx goes from 222 to 225 tests. DocxVerticalSpacingTest gains a table's two
edges landing on the paragraphs around it, a row's padding doing the same, and a table with
nothing above it keeping the bottom edge while losing the top.

Lane: canonical — document.backend.semantic.docx only. No public API change.

Page 2 is not touched by this and still drifts: Checklist sits 23.8pt high and the Lato
paragraph 37.4pt. That page opens with an image, so the next thing to measure is what an
image block reserves.