feat: a section numbers its pages in its own format

A roman front matter before a decimal body is one page style before another
in ODF and one w:pgNumType per section in Word, not a document setting — read
as one, its pages printed 1, 3 where the reference prints i, iii. HfSet grows
pageNumberFormat, both exports write it back, and the DOCX importer takes the
document's own format from the first section, as it already does the paper.

tests/roundtrip.test.ts

@@ -1853,6 +1853,26 @@ describe('Leg 12: per-section page margins (ODT + DOCX)', () => {
     const back = res.hfSections?.[1]?.margins ?? null;
     check('section margins round-trip', JSON.stringify(back) === JSON.stringify(wide), back);
   });
+
+  // A roman front matter before a decimal body: the format is the section's, not the
+  // document's, in both formats (ODF style:num-format, Word w:pgNumType w:fmt).
+  const romanSections = [{ ...EMPTY_HF_SET }, { ...EMPTY_HF_SET, pageNumberFormat: 'i' as const }];
+
+  it('ODT: the section layout carries style:num-format', async () => {
+    const bytes = await buildOdt(secDoc, margins, 'portrait', { sections: romanSections, pageCount: 2 });
+    const styles = strFromU8(unzipSync(bytes)['styles.xml']);
+    check('the minted layout is roman', /style:name="[^"]*Sec2"[^>]*>\s*<style:page-layout-properties[^>]*style:num-format="i"/.test(styles.replace(/\n/g, '')), styles.slice(0, 300));
+    const res = importOdt(bytes);
+    check('document format unchanged', res.pageNumbering?.format === '1', res.pageNumbering);
+    check('section format round-trips', res.hfSections?.[1]?.pageNumberFormat === 'i', res.hfSections?.[1]);
+  });
+
+  it('DOCX: the later sectPr carries w:pgNumType w:fmt', async () => {
+    const bytes = await buildDocx(secDoc, margins, 'portrait', { sections: romanSections, pageCount: 2 });
+    const res = importDocx(bytes);
+    check('document format unchanged', res.pageNumbering?.format === '1', res.pageNumbering);
+    check('section format round-trips', res.hfSections?.[1]?.pageNumberFormat === 'i', res.hfSections?.[1]);
+  });
 });
 
 describe('Leg 13: document properties (meta.xml / docProps/core.xml)', () => {