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.

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