List active experiments with changesets

Returns all active experiments for the authenticated environment and unit type, including group definitions and full changeset operations. Designed for server-side SDKs that cache experiment structure and apply changesets locally after resolving assignments via the bulk endpoint. Supports hash-based change detection: send the previous hash as ?hash= (or via If-None-Match) to receive 304 when the catalog is unchanged. Private experiments are returned to secret keys only.