Get State Transition Stats

A source state's per-arm transition statistics for the optimizer.

Every folded instance has source_partition_version == outcome_partition_version == partition_version — only transitions measured under one partition revision are compared. partition_version is the version filtered on (the org's current version when the caller passes none); None only when the org has no saved partition, in which case nothing is version-matched and the legacy/route mass lives in unversioned_member_count. unversioned_member_count is the graded cohort on NULL-version instances (route / seed / pre-versioning), reported so the caller sees legacy mass exists but it never pollutes the versioned arms.

excluded_day_count is the count of distinct actively excluded cohort days the fetch window contained — their instances are dropped before the version gate and every fold, reported so the caller sees the mass exists; it never enters the arms or the unversioned bucket. The rows stay stored, so revoking the exclusion restores them.