test(e2e): add SHA256-verified dataset downloader + EuRoC registry entry

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

scripts/download_dataset.py

@@ -0,0 +1,28 @@
+"""CLI: python scripts/download_dataset.py euroc_mh01"""
+
+import sys
+from pathlib import Path
+
+from gps_denied.testing.download import DATASET_REGISTRY, download_dataset
+
+
+def main(argv: list[str]) -> int:
+    if len(argv) != 2:
+        print(f"usage: {argv[0]} <dataset_name>", file=sys.stderr)
+        print(f"available: {', '.join(DATASET_REGISTRY)}", file=sys.stderr)
+        return 2
+    name = argv[1]
+    if name not in DATASET_REGISTRY:
+        print(f"unknown dataset: {name}", file=sys.stderr)
+        return 2
+    spec = DATASET_REGISTRY[name]
+    root = Path("datasets") / spec.target_subdir
+    dest = root / Path(spec.url).name
+    print(f"→ {dest}")
+    download_dataset(spec, dest)
+    print("OK")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))