fix(docx): keep a block on one page in Word when the layout does

[DemchaAV]

Why

keepTogether() and keepWithNext() are resolved by LayoutCompiler:

• A block that does not fit what is left of a page, but fits a fresh one, moves down whole.
• A keepWithNext block moves down together with the first line of the block after it.

Word re-paginates a DOCX itself and the export told it neither. So a card the page holds together split across Word's page break, and a heading the page moves down with its body could be left at the foot of the page above.

What changed

• DocxSemanticBackend.writeNode notes where the document body ends, writes the node, then hands everything the node wrote to keepOnOnePage:
  • w:keepLines on every paragraph.
  • w:keepNext on every paragraph but the last, which chains the block into one unit.
  • For keepWithNext, w:keepNext on the last paragraph too.
  • A table inside the block takes part row by row. Word reads keep-with-next on a row's paragraphs as keeping that row with the next one, so the last row ends the block like a last paragraph.
• Only a block the layout placed on one page is kept (DocxLayoutMetrics.onOnePage: start page equals end page). The layout keeps a block together only when a page can hold it and lets a taller one flow. A block that ran over a page break is that taller block, so it is left to flow in Word too. With no layout there is nothing to go on, and nothing is written.
• The check is generic over DocumentNode.keepTogether() / keepWithNext(), so it covers sections, modules and timelines. A LineNode with keepWithNext is dropped by this backend already, so it writes nothing.
• Recipe row for sections and containers, a new capability-matrix row, and a CHANGELOG entry under v2.5.0.

Verification

In an editor. A 300×300 page with filler paragraphs above a six-line section, converted with LibreOffice (soffice --headless --convert-to pdf), card lines by page:

Filler	Without keepTogether	With keepTogether
16	lines 0–3 on page 1, lines 4–5 on page 2	all on page 2
18	lines 0–1 on page 1, lines 2–5 on page 2	all on page 2

The layout places the section the same way: pages 0–1 without the flag, page 1 with it.

Gate. Full reactor gate (clean verify over core, render-pdf/docx/pptx, templates, testing, qa, coverage) → BUILD SUCCESS. Examples after install: 93 green. No committed preview moved.

New DocxKeepTogetherTest (6 tests):

• A kept block chains its paragraphs, leaves its last free, and leaves the next paragraph untouched.
• A keepWithNext heading keeps with its body.
• A plain section gets nothing.
• A kept block taller than the page gets nothing.
• A table inside a kept block chains row by row, and its last row ends the block.
• A bare export without a layout gets nothing.

Sabotaging the one-page check and the last-paragraph exception fails 4 of the 6.

Lane: shared-engine (render-docx) — paragraph properties only; no public API change. Touches DocxLayoutMetrics and the CHANGELOG head next to #724, so whichever merges second needs a rebase.