Starts an asynchronous computation of the benchmark's prompt embedding map.

Restricted to platform staff. Computing a map is what decides whether a benchmark exposes one on the anonymous read route, so it is a curation call rather than a benchmark-owner capability — a benchmark's owner cannot trigger or re-trigger it.

Returns a tracking id immediately; poll GET /benchmark/{benchmarkId}/prompt-embedding-map/status for progress. Responds with 409 Conflict when no embedding provider is configured, or when a computation for this benchmark is already in flight. Any map already being served stays in place until the new one succeeds, so triggering a recomputation is safe.