Census: MSH: (MeSH's synonym in GO's registry) is unfolded

[realmarcin]

From the Codex review of #244, after five internal rounds.

Ten pinned ProteinTraitsMech records carry structured MSH: cross-references (for example MSH:D041742). MSH is MeSH's listed synonym in GO's db-xrefs registry, and neither P nor norm includes it. The census's ProteinTraitsMech MESH count is therefore 4 instead of 14, against #244's stated rule.

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