net: chat client - async receive + OSK send over the link port

The application layer of the link-port demo, and it ties the whole system
together: the LCD terminal displays, the on-screen keyboard types, and the
link port carries a live chat.

Kernel: sys_srecv_nb (non-blocking link receive; A=byte, CF=none) and
sys_pollin (poll the OSK for a typed char without blocking) - syscalls
31/32. Both are what a poll loop needs to receive and type at once.

Userland: c/chat.c runs a poll loop - it feeds non-blocking bytes through a
SLIP receive state machine and prints whole incoming frames as messages,
while pollin() drives the on-screen keyboard; SELECT shows the keys, type a
line, START sends it as a frame. libc srecv_nb()/pollin().

Host: tools/gateway.py --mode chat is a simple bot peer (echoes each GB
message and injects a few async ones); --keys can drive the OSK for tests.

Verified: the gateway pushes 'welcome', '<alice> hey gameboy!', '<bob> nice
link cable' unprompted and the GB displays all three (async receive); typing
'hi' on the OSK echoes it and emits the SLIP frame \xC0hi\xC0 (send). A Game
Boy in the chat, keyboard on screen, over the link cable.

1 files changed, 2 insertions, 0 deletions

diff --git a/c/gbos.h b/c/gbos.h
index d207a75..df52fbe 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -15,6 +15,8 @@ unsigned char getpid(void);
 unsigned char pipe(void);         /* -> read fd; write end is read+1 */
 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 */
 
 /* helpers (c/libc.c) */
 void          putu(unsigned int n);              /* print decimal          */