fix: correct LST_DIAG_CD docs, TAF subtype keys, RUCC grouping, ZIP sources
- LST_DIAG_CD is not auto-created for MAX; must be constructed from DIAG_CD_* columns
- TAF gather_bene_level_diag_ndc_codes() creates LST_DIAG_CD on base_diag_codes, not base
- Fix RUCC grouping: codes 1-3 metro, 4-9 non-metro (not 1-7 metro)
- Fix ZIP centroids source: SimpleMaps/GeoNames, not NBER