Save an explicit named residual scenario

Requires API-key or first-party session authentication and a current account in the default-off server cohort. Valid current membership does not require a new paid subscription. Existing API quotas still apply. Responses are private, no-store. No query parameters are used by this operation; its inputs are the strict JSON body. This contract does not indicate source activation, deployment or an SDK release. Retained objects are scoped to the current account and their creator; no caller-supplied account/user grant is accepted. Current source rights are checked for every read/use, independently of archived grants. Creates an immutable base/downside/upside revision linked to an owned readable run. A parent revision must belong to the same run and creator. Formula uses fixed-dollar profit and purchase-independent carry; no live values/default costs are invented. Negative residuals remain explicit. The JSON object is strict, bounded to 16,384 bytes, and costs must contain each of the five buckets once.