feat: a cell's vertical alignment survives saving

Import read style:vertical-align but neither exporter wrote it, so a
mid-cell callout label jumped to the row's top on every save. odf-kit's
CellOptions and docx's TableCell both take it natively; DOCX import maps
w:vAlign in as well.

tests/roundtrip.test.ts

@@ -507,6 +507,7 @@ describe('Leg 1b: merged table cells (colspan/rowspan)', () => {
       ] },
     ],
   };
+  mergedDoc.content![1].content![0].content![1].attrs!.verticalAlign = 'middle';
 
   it('exports spans + covered cells and re-imports them', async () => {
     const bytes = await buildOdt(mergedDoc, margins, 'portrait');
@@ -527,6 +528,8 @@ describe('Leg 1b: merged table cells (colspan/rowspan)', () => {
     check('C has rowspan 2', rows[1]?.content?.[0]?.attrs?.rowspan === 2, rows[1]?.content?.[0]?.attrs);
     check('A shading round-trips (#FFFF00)', rows[0]?.content?.[0]?.attrs?.backgroundColor === '#FFFF00', rows[0]?.content?.[0]?.attrs?.backgroundColor);
     check('B has no shading', rows[0]?.content?.[1]?.attrs?.backgroundColor == null, rows[0]?.content?.[1]?.attrs?.backgroundColor);
+    check('B keeps vertical-align middle', rows[0]?.content?.[1]?.attrs?.verticalAlign === 'middle', rows[0]?.content?.[1]?.attrs?.verticalAlign);
+    check('A stays top-aligned', rows[0]?.content?.[0]?.attrs?.verticalAlign == null, rows[0]?.content?.[0]?.attrs?.verticalAlign);
 
     const textOf = (cell: N) => cell?.content?.[0]?.content?.[0]?.text;
     check('A text preserved', textOf(rows[0]?.content?.[0]) === 'A', textOf(rows[0]?.content?.[0]));

tests/unit/cell-valign.test.ts

@@ -0,0 +1,30 @@
+// A cell's verticalAlign attr round-trips through DOCX (w:vAlign); the ODF leg
+// (style:vertical-align) rides along in roundtrip.test.ts.
+import { describe, it, expect } from 'vitest';
+import { buildDocx } from '../../src/lib/export/docx';
+import { importDocx } from '../../src/lib/import/docx';
+
+type N = any;
+
+const cell = (text: string, verticalAlign?: string): N => ({
+  type: 'tableCell',
+  attrs: { colspan: 1, rowspan: 1, colwidth: [120], ...(verticalAlign ? { verticalAlign } : {}) },
+  content: [{ type: 'paragraph', content: [{ type: 'text', text }] }],
+});
+
+describe('table cell vertical alignment', () => {
+  it('round-trips middle/bottom and leaves top unset', async () => {
+    const doc: N = {
+      type: 'doc',
+      content: [
+        { type: 'paragraph', content: [{ type: 'text', text: 'Before' }] },
+        { type: 'table', content: [
+          { type: 'tableRow', content: [cell('A', 'middle'), cell('B', 'bottom'), cell('C')] },
+        ] },
+      ],
+    };
+    const res = importDocx(await buildDocx(doc, { top: 2, bottom: 2, left: 2, right: 2 }, 'portrait'));
+    const cells = (res.content.content!.find((n: N) => n.type === 'table') as N).content[0].content;
+    expect(cells.map((c: N) => c.attrs?.verticalAlign ?? null)).toEqual(['middle', 'bottom', null]);
+  });
+});