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, 22 insertions, 0 deletions

diff --git a/src/net.asm b/src/net.asm
index 939b975..f2c4279 100644
--- a/src/net.asm
+++ b/src/net.asm
@@ -39,3 +39,25 @@ sys_srecv::
 .got
     ld a, [rSB]
     ret
+
+; sys_srecv_nb() -> A = byte, CF set if none available (non-blocking).
+sys_srecv_nb::
+    ld a, $80
+    ld [rSC], a
+    ld a, [rSC]
+    bit 7, a
+    jr z, .got
+    scf
+    ret
+.got
+    ld a, [rSB]
+    and a                       ; CF = 0 (got a byte)
+    ret
+
+; sys_pollin() -> A = char typed on the OSK this poll, or 0 if none.
+sys_pollin::
+    call pad_poll
+    call osk_handle             ; CF set + A = char if a key was pressed
+    ret c
+    xor a
+    ret