- Add comprehensive documentation header explaining the structure's role
as single source of truth for neighbor cell information across multiple
protocols (NGAP/XnAP handover, measurement config, DU validation, SIB3/SIB4)
- Add detailed field comments documenting protocol-specific mappings:
* Core identification fields (gNB_ID, nrcell_id, physicalCellId)
* Frequency/RF parameters (absoluteFrequencySSB, subcarrierSpacing, band)
* Handover target identification (plmn, tac)
- Document NGAP/XnAP IE mappings for plmn and tac fields