net: SLIP framing over the link port + a host gateway (echo round-trip)

First step of link-port networking. The LCD terminal + OSK freed the serial
port from console duty, so it can be the network link.

Kernel (src/net.asm): raw link-port serial that bypasses the console/
terminal - sys_ssend (transmit, GB drives the clock) and sys_srecv (receive,
GB slave, blocks by yielding). Syscalls 29/30.

Userland: libc ssend()/srecv(); c/necho.c does SLIP (RFC 1055) framing over
them - send a packet, receive the reply, print it.

Host: tools/gateway.py wraps the emulator, owns its link serial, speaks SLIP,
and (for now) echoes every frame back - the "link cable adapter". Console
(ASCII) bytes on the same channel are printed for visibility.

Verified: `necho` sends a SLIP frame, the gateway decodes+echoes it, and gbos
prints the reply - a real framed round-trip over the Game Boy link port.
Next: swap the echo for actual network ops (DNS/HTTP or IRC/chat).

1 files changed, 14 insertions, 1 deletions

diff --git a/c/libc.s b/c/libc.s
index ec59bb7..50d7860 100644
--- a/c/libc.s
+++ b/c/libc.s
@@ -1,6 +1,6 @@
 	.module libc
 	.globl _writes, _putc, _puts, _nl, _strlen, _readc, _sexit, _getpid, _getargs
-	.globl _open, _close, _fgetc, _fputc, _flist, _fremove, _pipe
+	.globl _open, _close, _fgetc, _fputc, _flist, _fremove, _pipe, _ssend, _srecv
 	;; SDCC sm83 sdcccall(1): 1st arg -> A (byte) or DE (pointer); ret A / BC.
 	;; rst $30 (the trap) clobbers BC/DE/HL; SDCC treats them caller-saved, so
 	;; wrappers need not preserve them - but we compute cleanly regardless.
@@ -92,6 +92,19 @@ _pipe:
 	rst	#0x30
 	ret
 
+	;; void ssend(unsigned char b /*A*/)  -- link-port transmit
+_ssend:
+	ld	e, a
+	ld	c, #29			; SYS_SSEND
+	rst	#0x30
+	ret
+
+	;; unsigned char srecv(void) -> A  -- link-port receive (blocks)
+_srecv:
+	ld	c, #30			; SYS_SRECV
+	rst	#0x30
+	ret
+
 	;; unsigned char open(const char *name /*DE*/, unsigned char mode /*A*/) -> A(fd)
 _open:
 	ld	b, a			; B = mode