Pin (or unpin) the sampling values every generation with this voice inherits.

VoiceLabs (long-form phase 1). This is the "per-project pinning" a long-form creator needs: pick a seed and temperature once for the narrator voice and every chapter renders with the same sampling character instead of drifting take to take. A request that names its own seed/temperature still wins.

Its own sub-resource rather than fields on PUT /profiles/{id} because that route takes VoiceProfileCreate, where an omitted Optional field is indistinguishable from "clear this". Here both fields are required and nullable, so null unambiguously unpins.