Viewstamped Replication-backed Concord engine.

Commands use a stable VSR client session. Queries use quorum-confirmed read barriers, so they remain linearizable without appending to the replicated log or sharing the client's one-outstanding-command limit.

Replicated commands are recursively validated before contacting the VSR runtime. Commands containing functions, PIDs, ports, or references return {:error, {:invalid_command, reason}} and are never submitted to the log. The corresponding reasons are :function_in_spec, :pid_in_spec, :port_in_spec, and :ref_in_spec.

Queries are admitted through a fixed schema before a read barrier is issued. Unsupported shapes return {:error, :unsupported_query}; unsafe terms return {:error, {:invalid_query, reason}}.