Rolling 12-Month Forecast Update

when FP&A Analyst triggers a rolling forecast cycle dropping the closed month and appending the next out-month via POST /v1/forecast-cycles

negative) when a business unit attempts to modify an imported actual period then 403 with error_code: actuals_immutable.

Given a closed GL period with imported actuals;
when FP&A Analyst triggers a rolling forecast cycle dropping the closed month and appending the next out-month via POST /v1/forecast-cycles;
then a new 12-month forward window is created with prior actuals immutable, driver-based lines recalculate within 5 seconds of any input change, and the cycle blocks publishing until all business-unit sections are marked submitted;
(negative) when a business unit attempts to modify an imported actual period then 403 with error_code: actuals_immutable.

forecast_cycle { id: fc_*, window_start: date, window_end: date, status: enum(OPEN/SUBMITTED/PUBLISHED), external_id: string };
forecast_line { id: fl_*, cycle_id: string, account_code: string, entity_id: string, period: date, amount_minor: int64, currency_code: char(3), is_actual: bool, external_id: string };
driver_input { id: di_*, cycle_id: string, driver_name: string, value: numeric, period: date, external_id: string };
(reference, product may differ).

POST /v1/forecast-cycles { window_start, window_end, external_id } -> 201 { id, status: OPEN };
PUT /v1/forecast-cycles/{id}/driver-inputs { driver_name, value, period };
POST /v1/forecast-cycles/{id}/publish;
emits forecast.cycle_published event;
idempotent via external_id.

OPEN -> SUBMITTED -> PUBLISHED;
guard: PUBLISHED blocked until all business-unit sections status=SUBMITTED;
actual periods immutable once imported;
prior 24 cycles retained.