Create a bank securities classification

Fresh verified non-API-key staff with ledger:securities:write required. Proposes the next version of an approved instrument's classification: designation (available_for_sale; held_to_maturity is refused 422 because the program's accounting policy admits AFS only), rcb_line (a retained FFIEC 051 Schedule RC-B body item: 1, 2, 3, 4a1, 4a2, 4b1, 4b2, 4b3, 4c1a, 4c1b, 4c2a, 4c2b, 5a, 5b, 6a or 6b), rate_type (fixed or floating), structured_note (items 2, 3, 5 and 6 only), effective_date, evidence and rationale. A Treasury bill, the only instrument kind, is refused 422 unless it is item 1, fixed rate and not structured. The version binds the instrument's exact content digest; a new version must take effect after the approved one and is refused 409 while another awaits review. The database refuses the same.