Define the composite engine interface and a newEngine() factory keyed by a per-engine layout table, so the agent's defaults, health probe and command implementation all come from the engine the image bakes rather than from PostgreSQL-shaped constants. Rename PGBin/PGData/PGUser to EngineBinDir/EngineDataDir/EngineUser, delete PGIsReady, and give engineProbe an injected state function with the pg_isready exit-code mapping moved into the PostgreSQL implementation.
setup.sh now stamps /etc/spinifex-rds/engine and buildAgentUserData emits RDS_ENGINE. The agent compares the two after registering and refuses to run when they disagree, reporting unhealthy before the bootstrap handoff is written so rds-init never unblocks and no datadir is touched.
Julian Sommer