feat(producer): add DecodedPacket marker

[lspgn]

Summary

• Add a DecodedPacket marker interface to constrain producer inputs to known decoded packet types (NetFlow v5/v9/IPFIX, sFlow) while keeping flexibility.
• Use marker methods on packet structs to enforce compile-time type safety without adding behavior.

Rationale

• Marker interface pattern provides compile-time checks in place of interface{} without forcing a shared behavior contract.