test(e2e): rename registry entry to euroc_machine_hall with real SHA256

The prior registry entry was speculative: ``euroc_mh01`` pointing at an
old ``robotics.ethz.ch`` URL that no longer resolves (TCP timeout).
The dataset moved to ETH Research Collection (DOI 10.3929/ethz-b-000690084)
as a single 12.6 GB ``machine_hall.zip`` bundle containing MH_01…MH_05.
There's no stable direct download URL — DSpace gates behind a UI —
so:

- Renamed entry: ``euroc_mh01`` → ``euroc_machine_hall`` (matches the
  actual artifact).
- SHA256 set to the real bundle hash 5ed7d07…
- URL left empty (same pattern as ``vpair_sample``); the CLI now
  exits 3 and prints fetch instructions for empty-URL entries instead
  of crashing on ``urllib.request.urlretrieve("")``.
- Adapter ``DatasetNotAvailableError`` message and conftest skip-reason
  updated to tell engineers how to fetch/unpack manually.
- ``test_registry_has_euroc_machine_hall`` pin test replaces the old
  pin; asserts real hash (not the ``"0"*64`` placeholder).

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

scripts/download_dataset.py

@@ -1,4 +1,10 @@
-"""CLI: python scripts/download_dataset.py euroc_mh01"""
+"""CLI: python scripts/download_dataset.py <dataset_name>
+
+Registered datasets: see DATASET_REGISTRY in gps_denied.testing.download.
+Entries with an empty URL are manual-download-only (e.g. EuRoC via ETH
+Research Collection, VPAIR via a Zenodo form) — this script prints the
+instructions and exits instead of guessing a download URL.
+"""
 
 import sys
 from pathlib import Path
@@ -14,8 +20,23 @@ def main(argv: list[str]) -> int:
     name = argv[1]
     if name not in DATASET_REGISTRY:
         print(f"unknown dataset: {name}", file=sys.stderr)
+        print(f"available: {', '.join(DATASET_REGISTRY)}", file=sys.stderr)
         return 2
     spec = DATASET_REGISTRY[name]
+    if not spec.url:
+        print(
+            f"Dataset '{name}' has no automated download URL.",
+            file=sys.stderr,
+        )
+        print(
+            "Fetch manually, then place the archive so its SHA256 matches:\n"
+            f"  expected sha256: {spec.sha256}\n"
+            f"  expected path subdir under datasets/: {spec.target_subdir}\n"
+            "See DATASET_REGISTRY comments in src/gps_denied/testing/download.py "
+            "for per-dataset instructions.",
+            file=sys.stderr,
+        )
+        return 3
     root = Path("datasets") / spec.target_subdir
     dest = root / Path(spec.url).name
     print(f"→ {dest}")