Cut 12.1.0 and bump the package version in pyproject.toml, uv.lock, and
CITATION.cff.

Minor: one new feature and one fix since 12.0.0, no breaking changes —
ManualProvenance.upstream_source_record_urls rehomes source record URLs
onto per-upstream supporting_data_source entries (#104), and p-value
columns emit in scientific notation again (#102). Also ships the docs
correction that stops prescribing pipe as the separator for multivalued
slots (#103).