Bulk Auto Assign Variants

Let an agent place up to 20 variants into every fitting region.

One batched LLM call decides coverage subtractively per approved variant — every region minus the ones it excludes for a concrete reason — then launches a region experiment for each remaining region via LaunchRegionExperimentUseCase. When approve_first is set, draft variants are approved first (same request). One REGION_EXPERIMENT_LAUNCHED audit row per launched assignment, plus one VARIANT_APPROVED per real approval.