Dissector for NETLINK on LINUX_SLL/SLL2 (ARPHRD_NETLINK) #2500

Summary

Part of #2460. LinuxSLL/LinuxSLL2 fall back to Raw for the NETLINK
arphrd_type (824), whose protocol_type field isn't an EtherType (see
_ARPHRD_NOT_ETHERTYPE in pcapkit/protocols/link/linux_sll.py). The header
is preserved, nothing crashes, but nothing is dissected either.

Scope

Linux kernel Netlink socket framing is a different framing entirely from
"next protocol over this link" -- it is not a protocol layered over
Ethernet/Frame-Relay/etc., it's the kernel's own IPC transport, captured
whole on an nlmon interface. No Netlink dissector exists in this codebase.

A dissector would need:

• the struct nlmsghdr framing (len, type, flags, seq, pid), 16
  octets, repeating for every message in the capture buffer (Netlink packs
  several messages per read)
• per-family payload parsing is a separate, much larger question (routing,
  NETLINK_GENERIC, etc.) and is out of scope for a first slice -- the
  header alone, with the payload kept as Raw, is the reasonable first cut
• authoritative source: RFC 3549 (Netlink as an IPC mechanism) plus the
  Linux kernel's own <linux/netlink.h>

Not implemented here -- tracked as its own issue per #2460's scoping
("each of these is effectively its own new-protocol implementation, not a
tweak").