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>

src/gps_denied/testing/download.py

@@ -37,6 +37,13 @@ DATASET_REGISTRY: dict[str, DatasetSpec] = {
         sha256="0" * 64,  # placeholder — see note above
         target_subdir="euroc/MH_01",
     ),
+    "vpair_sample": DatasetSpec(
+        url="",  # manual download only — see Zenodo link on
+                 # https://github.com/AerVisLoc/vpair
+        sha256="c83f41fa7c9f39debe4898abceb009bff8a2b17b4840defb1831ecf127a9bf6b",
+        target_subdir="vpair/sample",
+        unpack=True,
+    ),
 }