Decode system Nexus worker payloads

[VegetarianOrc]

Summary

• Select the system Nexus payload converter for marked worker inputs after nested external retrieval and codec decoding.
• Rename the Nexus payload serializer to reflect its real responsibilities.
• Add focused coverage for marked inputs, nested codec processing, unmarked inputs, and existing payload validation behavior.

Testing

• poe test -s -k "nexus_worker or system_nexus or payload_validation" (23 passed)
• poe test -s -k "nexus_payload_serializer or payload_validation" (8 passed)
• poe format
• poe lint