feat: libc - add SYS_GETCHAR_NB and a non-blocking stdin readiness probe

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NQRTGoYgnZQsh7uCh6Jsxm

programs/man/syscalls.2

@@ -233,6 +233,12 @@
     Block until a printable character is typed.
         char montauk::getchar();
 
+.B SYS_GETCHAR_NB (166)
+    Return the next printable character, or 0 when none is
+    pending. Never blocks; key events carrying no ascii are
+    drained rather than left to stall a following getchar.
+        char montauk::getchar_nb();
+
 .B SYS_INPUT_WAIT (123)
     Block until the input serial number differs from
     observedSerial or the timeout elapses; used to sleep