net: UDP sockets in the kernel + a DNS resolver (real hostname lookups)

Adds UDP to the kernel socket layer on top of the ICMP core:
- net_sum() (raw folded sum) split out of net_cksum() so a UDP pseudo-header
  (src/dst IP + proto + length) can seed the segment checksum.
- udp_send: builds IP+UDP with the pseudo-header checksum; NET_BIND sets the
  local/source port; net_connect sets the peer.
- udp_in: demuxes inbound UDP by destination port to the bound socket
  (net_find_udp), delivers the payload + source addr.

Also fixes a real recv bug: net_pump clobbers BC/DE/HL, so the old recv timeout
counted in registers and was effectively random. recv now counts in WRAM.

New `nslookup HOST` (PROG_NSLOOKUP=28, bank 30): builds a DNS A query and parses
the answer (with 0xC0 name-compression) entirely in userland over a UDP socket -
the kernel never sees DNS, just UDP. Verified through the bridge NAT:
    /# nslookup example.com   ->  example.com -> 172.66.147.243

This gives us name resolution for the TCP/HTTP demo next.

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