Census: TC: (TCDB's own prefix) is still unfolded

[realmarcin]

From the round-3 review of #244 (follows #271).

ProteinTraitsMech writes TCDB transporter numbers two ways:

• identifier: TCDB:1.A…, counted (4,705);
• xrefs: - TC:2.A.1.10.1, dropped (194).

TC is Bioregistry's listed synonym for TCDB. That is the same relationship as DSM→DSMZ and TAXON→NCBITaxon, which #244 folds.

Fix: fold TC into TCDB in the census. TCDB has no heatmap column, so build_subsets is unaffected.