feat: a table keeps the cell margins its file gave it

The cell padding was a constant in three places that disagreed with each
other, so a narrow column lost 0.38cm it did not have: a bibliography
marker column of 0.71cm wrapped "[2] " onto three lines and every entry
grew with it, costing a fixture a whole page.

The table now carries its own margins (w:tblCellMar, direct or from the
table style; fo:padding on the ODF cell style) and both exports write
them back. A side within a twentieth of a millimetre of the default is
the default, or Word's 108 twips would tag every table.

tests/unit/cell-padding.test.ts

@@ -4,6 +4,10 @@
 import { describe, it, expect } from 'vitest';
 import { unzipSync, strFromU8 } from 'fflate';
 import { buildOdt } from '../../src/lib/export/odt';
+import { importOdt } from '../../src/lib/import/odt';
+import { buildDocx } from '../../src/lib/export/docx';
+import { importDocx } from '../../src/lib/import/docx';
+import { cellPaddingAttr } from '../../src/lib/editor/extensions/tableCellPadding';
 
 type N = any;
 
@@ -37,4 +41,31 @@ describe('table cell padding', () => {
       expect(props).toMatch(/fo:padding-bottom="0cm"/);
     }
   });
+
+  it('collapses a producer-rounded default, keeps a real value', () => {
+    // Word's 108 twips and LibreOffice's 0.191cm are both our 0.19cm default.
+    expect(cellPaddingAttr([0, 0.1905, 0, 0.191])).toBeNull();
+    expect(cellPaddingAttr([0.026, 0.026, 0.026, 0.026])).toEqual([0.026, 0.026, 0.026, 0.026]);
+  });
+
+  it('round-trips a table’s own cell margins', async () => {
+    const tight: N = structuredClone(doc);
+    tight.content[1].attrs = { cellPadding: [0.03, 0.05, 0.03, 0.05] };
+    const bytes = await buildOdt(tight, { top: 2, bottom: 2, left: 2, right: 2 }, 'portrait');
+    const xml = strFromU8(unzipSync(bytes)['content.xml']);
+    expect(xml).toMatch(/fo:padding-left="0\.05cm"/);
+    expect(xml).toMatch(/fo:padding-top="0\.03cm"/);
+
+    const back = await importOdt(bytes);
+    const table = back.content.content!.find((n: N) => n.type === 'table') as N;
+    expect(table.attrs?.cellPadding).toEqual([0.03, 0.05, 0.03, 0.05]);
+
+    const docx = await buildDocx(tight, { top: 2, bottom: 2, left: 2, right: 2 }, 'portrait');
+    const backDocx = importDocx(docx);
+    const docxTable = backDocx.content.content!.find((n: N) => n.type === 'table') as N;
+    // Word measures in twips, so 0.05cm comes back a twentieth of a point off.
+    for (const [i, cm] of [0.03, 0.05, 0.03, 0.05].entries()) {
+      expect(docxTable.attrs?.cellPadding[i]).toBeCloseTo(cm, 2);
+    }
+  });
 });