Improve AF_XDP pipeline

[pgigis]

Summary

This PR simplifies the XDP pipeline by reducing coordination in the hot path, making drop/aggregate handling more scalable, and clarifying how packets move through ingress and forwarding paths.

Testing

• cmake -S . -B build && cmake --build build
• Unit tests (ctest --test-dir build) if available
• [] Other (describe):

Notes

• Behavior changes include clearer separation between raw_socket and raw_nic, with raw_nic now treated as a true L2 replay path.
• CLI defaults no longer silently override YAML configuration unless the corresponding flags are explicitly provided.
• Example configs and docs were updated to reflect the intended ingress/egress behavior more clearly.