Create an ISO Manual Adjustment (the same record produced by the /Firm/ISOManualAdjustmentsManagement page). Supports OnePeriod and ManyPeriods (including open-ended ranges with endReportPeriod=null); a negative amount reduces the payout. Returns HTTP 200 with the created adjustment. Field and business validation errors are returned together as HTTP 400 (validationErrors). Requires the iso-manual-adjustments-management module and the "Allow ISO Manual Adjustments Management API" role option (otherwise HTTP 403).