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, 30 insertions, 1 deletions

diff --git a/include/gbos.inc b/include/gbos.inc
index a80841c..a4c9d52 100644
--- a/include/gbos.inc
+++ b/include/gbos.inc
@@ -130,7 +130,36 @@ DEF SYS_SSEND   EQU 29   ; link-port: transmit one byte   (E=byte)
 DEF SYS_SRECV   EQU 30   ; link-port: receive one byte    (-> A=byte, blocks)
 DEF SYS_SRECVNB EQU 31   ; link-port: receive, non-blocking (-> A=byte, CF=none)
 DEF SYS_POLLIN  EQU 32   ; poll OSK for a typed char       (-> A=char or 0)
-DEF SYS_MAX     EQU 33
+DEF SYS_NET     EQU 33   ; socket layer                    (DE=&netreq -> A=result)
+DEF SYS_MAX     EQU 34
+
+; ---- socket layer (SYS_NET) --------------------------------------------------
+; netreq struct (in caller RAM; DE points to it):
+DEF NR_OP    EQU 0     ; operation (below)
+DEF NR_SOCK  EQU 1     ; socket id
+DEF NR_BUF   EQU 2     ; 2: pointer to payload buffer
+DEF NR_LEN   EQU 4     ; 1: payload length (in) / bytes received (out)
+DEF NR_IP    EQU 5     ; 4: IPv4 address
+DEF NR_PORT  EQU 9     ; 2: port (hi,lo)
+DEF NR_SIZE  EQU 11
+; ops
+DEF NET_SOCKET  EQU 0  ; NR_SOCK(type in) -> A=sockid or $FF
+DEF NET_CONNECT EQU 1  ; NR_SOCK, NR_IP, NR_PORT -> set peer
+DEF NET_SEND    EQU 2  ; NR_SOCK, NR_BUF, NR_LEN -> A=0/$FF
+DEF NET_RECV    EQU 3  ; NR_SOCK, NR_BUF, NR_LEN(max) -> A=len or $FF(timeout); fills NR_IP
+DEF NET_CLOSE   EQU 4  ; NR_SOCK
+DEF NET_BIND    EQU 5  ; NR_SOCK, NR_PORT -> bind local port (UDP)
+DEF NET_POLL    EQU 6  ; pump RX once (answer pings) -> A=0
+; socket types
+DEF SOCK_ICMP EQU 1
+DEF SOCK_UDP  EQU 2
+DEF SOCK_TCP  EQU 3
+DEF MAX_SOCKS EQU 4
+; SLIP framing (RFC 1055)
+DEF SLIP_END     EQU $C0
+DEF SLIP_ESC     EQU $DB
+DEF SLIP_ESC_END EQU $DC
+DEF SLIP_ESC_ESC EQU $DD
 ; SYS_KILL (9) is now implemented (B=pid -> A=0/$FF; pid 1 is immortal)
 ; (SYS_OPEN=4 / SYS_CLOSE=5 are now implemented by the filesystem)