pg.Cluster wraps a primary pool and an optional read-replica pool.
Routing is explicit — Read(), Write(), Primary() — with no SQL
inspection: statement text misroutes CTE writes and SELECT ... FOR
UPDATE in both directions.
An unhealthy replica falls back to the primary behind a ping-based
circuit that retries on a doubling backoff window, and migrations always
run through Write().
pg.ClusterDSNsFromEnv resolves both endpoints from the environment,
naming the read-only host explicitly rather than deriving it from the
primary's.
