test(super-editor): pin SD-3158 start/end-only tcMar round-trip (#3314)

Narrower variant of SD-3152: a w:tcMar with ONLY w:start/w:end (no
top/bottom). Exercises the vertical-side branch of the merge loop being
fully skipped, while the horizontal branch still has to preserve the
logical key family.

Fixture is built by the word-api ooxml-fixture SDK CLI
(build --type tc-mar-logical) and pre-validated with OpenXmlValidator
(Office2019 profile). Same source the fixture-feedback harness uses,
so the test exercises identical input.

The original SD-3158 report was a stale-dist false positive in
fixture-feedback (packages/superdoc/dist is gitignored and was pre-fix).
After rebuild, the SDK fixture exports cleanly. This test pins the
behavior so a future regression on the start/end-only branch won't slip
through CI.

Author: caio-pizzol

tests/behavior/tests/exporting/fixtures/sd-3158-tcmar-startend-only.docx

@@ -75,3 +75,35 @@ test('@behavior SD-3152: zero-edit round-trip preserves tcMar logical vs physica
     expect(/<w:end\b/.test(inner) && /<w:right\b/.test(inner)).toBe(false);
   }
 });
+
+// SD-3158: narrower variant — a cell with ONLY <w:start>/<w:end> tcMar, no
+// top/bottom. The SD-3152 fix needs to hold when the merge loop's vertical-side
+// branch is fully skipped (cellMargins.top and .bottom are undefined). Fixture
+// is generated by the word-api ooxml-fixture SDK CLI (build --type tc-mar-logical)
+// and pre-validated with OpenXmlValidator (Office2019 profile).
+const STARTEND_ONLY = path.resolve(__dirname, 'fixtures/sd-3158-tcmar-startend-only.docx');
+
+test('@behavior SD-3158: zero-edit round-trip of start/end-only tcMar emits only logical, no duplicate physical', async ({
+  superdoc,
+}) => {
+  await superdoc.loadDocument(STARTEND_ONLY);
+  await superdoc.waitForStable();
+
+  const bytes: number[] = await superdoc.page.evaluate(async () => {
+    const blob: Blob = await (window as any).editor.exportDocx();
+    const buffer = await blob.arrayBuffer();
+    return Array.from(new Uint8Array(buffer));
+  });
+
+  const outZip = await JSZip.loadAsync(Buffer.from(bytes));
+  const outXml = await outZip.file('word/document.xml')!.async('string');
+
+  const tcMarBlocks = Array.from(outXml.matchAll(/<w:tcMar\b[^>]*>([\s\S]*?)<\/w:tcMar>/g)).map((m) => m[1]);
+  expect(tcMarBlocks).toHaveLength(1);
+
+  const children = Array.from(tcMarBlocks[0].matchAll(/<w:(top|start|left|bottom|end|right)\b/g)).map(
+    (m) => `w:${m[1]}`,
+  );
+  // Source had ONLY w:start + w:end. Export must match: no top/bottom inserted, no physical pair added.
+  expect(children).toEqual(['w:start', 'w:end']);
+});