Identifier derivation shared by the semantic checks and the emitters.

IDL names surface in generated Erlang unchanged where the wire sees them (struct_info keys, enum atoms, field-name binaries); every derived identifier — record tags, type aliases, helper function names, constant macros — goes through snake/1, so an IDL InvoiceBankAccount becomes the readable invoice_bank_account instead of the lowercased mush invoicebankaccount.