test(e2e): register vpair_sample SHA256 in dataset registry

URL left empty because VPAIR sample is form-gated on Zenodo.
Registry records the known-good SHA256 for manual downloads; the
download_dataset() helper refuses empty URLs so this cannot be used
to auto-fetch a changed artifact.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/e2e/test_download.py

@@ -19,6 +19,15 @@ def test_registry_has_euroc():
     assert len(spec.sha256) == 64
 
 
+def test_registry_has_vpair_sample():
+    assert "vpair_sample" in DATASET_REGISTRY
+    spec = DATASET_REGISTRY["vpair_sample"]
+    # URL intentionally empty — Zenodo form-gated download; registry records SHA256 only.
+    assert spec.url == ""
+    assert len(spec.sha256) == 64
+    assert spec.sha256 != "0" * 64  # real hash, not the placeholder
+
+
 def test_verify_sha256_matches(tmp_path: Path):
     data = b"hello world"
     f = tmp_path / "x.bin"