test(lbug): exercise the BufferedCSVWriter FLUSH_EVERY boundary in vitest (#2215 review)

The U3 addRow change (returns a flush promise only on flush; undefined when
buffered) and the loop's `if (pending) await pending` were only crossed by the
bench, never vitest (all fixtures are <500 nodes). Add a 600-node graph through
streamAllCSVsToDisk asserting all rows land exactly once across the 500-row
flush boundary — no drops, dups, or corruption.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gitnexus/test/integration/csv-pipeline.test.ts

@@ -253,6 +253,27 @@ describe('streamAllCSVsToDisk', () => {
     expect(fileCsv).toBeDefined();
     expect(fileCsv!.rows).toBe(1);
   });
+
+  it('crosses the BufferedCSVWriter FLUSH_EVERY boundary without losing rows', async () => {
+    // FLUSH_EVERY=500; a >500-node graph forces ≥1 mid-stream flush, exercising
+    // addRow's flush-promise return + the loop's `if (pending) await pending`
+    // path that the small fixtures above never reach (only the bench did).
+    const N = 600;
+    const nodes = Array.from({ length: N }, (_, i) => ({
+      id: `File:src/f${i}.ts`,
+      label: 'File' as const,
+      name: `f${i}.ts`,
+      filePath: `src/f${i}.ts`,
+    }));
+    const result = await streamAllCSVsToDisk(buildTestGraph(nodes), repoDir, csvDir);
+
+    const fileCsv = result.nodeFiles.get('File');
+    expect(fileCsv).toBeDefined();
+    expect(fileCsv!.rows).toBe(N); // no rows dropped/duplicated at the flush boundary
+    const dataRows = dataRowsOf(await fs.readFile(fileCsv!.csvPath, 'utf-8'));
+    expect(dataRows).toHaveLength(N);
+    expect(new Set(dataRows).size).toBe(N); // all distinct — no flush-boundary corruption
+  });
 });
 
 /**