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.

1 files changed, 6 insertions, 0 deletions

diff --git a/c/sock.h b/c/sock.h
index 9af2351..1298271 100644
--- a/c/sock.h
+++ b/c/sock.h
@@ -42,6 +42,12 @@ static unsigned char net_recv(unsigned char s, void *buf, unsigned char max) {
     _nr.op = 3; _nr.sock = s; _nr.buf = (unsigned char *)buf; _nr.len = max;
     return sys_net(&_nr);
 }
+/* set the local (source) port - needed so UDP replies demux back to us */
+static unsigned char net_bind(unsigned char s, unsigned int port) {
+    _nr.op = 5; _nr.sock = s;
+    _nr.port_hi = (unsigned char)(port >> 8); _nr.port_lo = (unsigned char)port;
+    return sys_net(&_nr);
+}
 static void net_close(unsigned char s) {
     _nr.op = 4; _nr.sock = s; sys_net(&_nr);
 }