fix(fake-tcp): normalize Linux packet sockets to layer 3 (#2255)

Use AF_PACKET SOCK_DGRAM so Ethernet, TUN, and point-to-point
interfaces expose the same IP payload to the BPF filter. Rebuild the
synthetic Ethernet envelope expected by fake TCP on receive and strip
it before transmitting through the cooked socket.

Bind sockets to the selected IP protocol and reject non-initial IPv4
fragments before reading TCP ports. Preserve peer MAC addresses on
Ethernet links.

Add privileged TUN and veth tests for IPv4/IPv6 receive, send, tuple
filtering, and fragment rejection, and enable them in Linux CI.

Co-authored-by: KKRainbow <443152178@qq.com>
Co-authored-by: Max Sum <4883681+Max-Sum@users.noreply.github.com>