feat(docx): give a word document the page it was written on

[andiwand]

🤖 Generated with Claude Code

A docx carried no page layout at all — text_root_page_layout returned an empty
struct, and nothing in the tree read w:sectPr. With text_document_margin on,
the renderer drew a page box that hugged the text: no page size, no margins, a
white rectangle the width of the longest line. Every other paginated format
already had one (odt/odp/odg off the master page, pptx off p:sldSz, .ppt off
the DocumentAtom, pdf off the MediaBox).

Document now reads the section properties in its constructor, the way the pptx
Document computes its slide layout: w:pgSz for size and orientation, w:pgMar
for the margins. A negative w:top/w:bottom — the signed form that lets a header
flow into the body — clamps to zero, since as a CSS margin it would push the text
off the page.

Which w:sectPr. The first in document order. The model carries one layout
per text root while Word carries one per section, and w:body/w:sectPr describes
the last section — a section is closed by the w:pPr/w:sectPr of its final
paragraph. The first one is therefore the page the document opens with, and for
the single-section documents that are the norm the two coincide. sample3.docx
has two sections with different bottom margins, so the new test pins the choice.

odr-page-outer" style="width:8.5in;min-height:11in;"
odr-page-inner" style="margin-right:1in;margin-top:1in;margin-left:1in;margin-bottom:0.5in;"

Reference output moves for the 24 docx documents and nothing else; each diff is
those two lines.

README.md claimed "page layout (via master page)" — docx has no master pages
and it was never implemented. .doc has the same gap and keeps it: section
properties there mean PlcfSed → Sepx, real work in a module that does visible
text only, so it is written down in oldms/text/AGENTS.md instead.