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

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NQRTGoYgnZQsh7uCh6Jsxm

kernel/src/Api/Syscall.cpp

@@ -134,6 +134,8 @@ namespace montauk::abi {
                 return 0;
             case SYS_GETCHAR:
                 return (int64_t)Sys_GetChar();
+            case SYS_GETCHAR_NB:
+                return (int64_t)Sys_GetCharNb();
             case SYS_PING:
                 return (int64_t)Sys_Ping((uint32_t)frame->arg1, (uint32_t)frame->arg2);
             case SYS_SPAWN: