lean-corpus-density pins the corpora its committed data was built from

The committed density data in lean-corpus-density now names the corpora and revisions it was built from (550a97d), so a figure in the report can be traced back to a specific corpus state instead of to the tool that read it, and the reproduction check of 21 September is recorded alongside it (31558d4). The repository’s claim is that machine mathematics accumulates; the data has to be pinned before that claim is checkable by anyone else.