net: proper socket API in the kernel (SYS_NET) - no more SLIP in userland

The network stack moves into the kernel. src/socket.asm owns SLIP framing,
IPv4, RFC1071 checksums, and ICMP; programs now speak a socket API through one
syscall (SYS_NET, DE=&netreq dispatched by op): net_socket/connect/send/recv/
close/poll (c/sock.h). No program touches SLIP, IP headers, or checksums.

- Socket table (4 sockets) + tx/rx buffers in WRAM0; our IP = 10.0.0.2.
- net_pump: drains the link, reassembles SLIP frames, demuxes IPv4. Inbound
  ICMP echo requests are auto-answered in-kernel, so the GB replies to pings
  whenever any process pumps RX.
- ICMP sockets: send() emits an echo request to the connected peer; recv()
  returns the matching reply (with a spin/yield timeout).

ping.c is now a ~15-line socket client; netd.c is just `for(;;){net_poll();
yield();}`. Verified over tunbridge:
    /# ping 1.1.1.1     -> replies from the real internet (kernel builds it all)
    host# ping 10.0.0.2 -> 4/4, 0% loss (kernel auto-answers)

Gotchas recorded: gbos.inc isn't a make dep (touch asm after editing); this
crt0 doesn't copy initializers (fill arrays at runtime); and the arg string at
0xA000 overlaps _DATA, so parse targets must sit past it (big buffer first).
UDP and TCP sockets build on this same core next.

1 files changed, 1 insertions, 0 deletions

diff --git a/c/gbos.h b/c/gbos.h
index df52fbe..6ecbd4a 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -17,6 +17,7 @@ void          ssend(unsigned char b);   /* link-port: transmit one byte */
 unsigned char srecv(void);              /* link-port: receive one byte (blocks) */
 int           srecv_nb(void);           /* link-port: receive, or -1 if none */
 unsigned char pollin(void);             /* poll OSK for a typed char, or 0 */
+unsigned char sys_net(void *req);       /* socket layer trap (see sock.h)  */
 
 /* helpers (c/libc.c) */
 void          putu(unsigned int n);              /* print decimal          */