Conditional prior baseline

The launch baseline that SN21 miners must beat to earn emissions in an epoch. Per episode it is the historical mean actual outcome for the same (campaign_type, action_type) over a rolling training window; per epoch a miner’s mean episode score must exceed this baseline or emissions are zero regardless of rank. The conditional prior is harder to game than a literal predict-zero floor (which trivial heuristics can clear) and forces measurable improvement over a transparent historical average. The baseline shifts toward platform system estimates as operator-managed episodes grow as a share of the pool; any change is announced ≥ 4 weeks ahead. Authoritative spec: SN21_REWARD_MECHANISM.md §"Component 1 — Participation gate".