Restrict holdout overlap check to holdout vs dev/full only

dev and full are allowed to share files — only holdout must be strictly
isolated. When creating dev or full, only compare against holdout.manifest;
when creating holdout, compare against all other manifests in the dir.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/bump_dataset_version.py

@@ -263,7 +263,13 @@ def check_holdout_overlap(
 
     new_set = set(resolved_new_files)
     overlaps: list[tuple[str, Path]] = []
-    for existing in sorted(manifest_dir.glob("*.manifest")):
+    # When creating holdout, check against all other manifests (dev, full, …).
+    # When creating dev/full, only check against holdout — dev vs full overlap is allowed.
+    if output_resolved.name == "holdout.manifest":
+        candidates = sorted(manifest_dir.glob("*.manifest"))
+    else:
+        candidates = [holdout_path]
+    for existing in candidates:
         if existing.resolve() == output_resolved:
             continue
         try: