Postgres engine: open constructor, PgOpts, pool + listener wiring — forwarder skeleton with the POC-pinned pitfalls structurally excluded, seam error mappings, wave-3 stub surface (task pg-engine-open-opts)

Cargo.lock

@@ -28,6 +28,7 @@ dependencies = [
  "alkstore",
  "alkstore-contract-suite",
  "deadpool-postgres",
+ "serde_json",
  "thiserror",
  "tokio",
  "tokio-postgres",