feat(transport): GTP-U N3 interface awareness for UPF co-location

[Yanstart]

Context

Standards alignment: TS 29.281, TS 23.501 §5.8.

For operator deployments where QUICK- is co-located with UPF, support GTP-U encapsulated traffic on the N3 interface.

References

• TS 29.281 (GTP-U)
• TS 23.501 §5.8.2.11
• NTN conformance: docs/standards/3GPP-ALIGNMENT.md §3.3