Early-buyer quality for up to 20 RHC tokens (BASIC+)

Scores several tokens' earliest-buyer cohorts in one call. The cap is 20, not the Solana 50 — RHC buyer-quality is a per-token cohort computation (ordered early-buyer scan + bundle RPC + alpha/cluster joins) that cannot collapse into one set-based query, so 50 would mean ~200 round-trips behind a single request. A per-token failure degrades to an error entry rather than failing the batch. Scores are identical to the single-token endpoint (shared scorer). Tier: BASIC.