Patch release. Closes a gap in 0.6.0 where the validator's
batch-citation check went through aexp.linking.list_batches() (local
signac project only) and missed matches that lived only in the
universal cross-machine ledger. The fix builds an
(experiment_id, condition) -> [job_ids] lookup from .aexp/ledger/
entries and counts those matches before the elsewhere/empty paths.

Surfaced during the electricrag laptop validate post-migration:
F005 batch citations spuriously emitted finding.empty_batch for 3
cluster-registered runs that were present in the just-pulled ledger
but absent from the laptop's local .runs/workspace/. After this fix,
those resolve cleanly.

Note: 0.6.0 was tagged in source but never published to PyPI; this
0.6.1 release is the first publish of the cross-machine-ledger
surface. PyPI history goes 0.5.0 -> 0.6.1.