feat: Rust std target + ruststd tests; SYS_EXIT_GROUP; fix mkdir/create under missing parent

kernel/src/Api/Syscall.hpp

@@ -380,6 +380,12 @@ namespace montauk::abi {
        sibling thread ends only that thread, so abort() needs its own call. */
     static constexpr uint64_t SYS_ABORT                = 198;  // () -> does not return
 
+    /* Normal termination of the whole process from any thread, with an exit
+       code (0..255) instead of SYS_ABORT's fixed TerminatedAborted. libc
+       exit() and Rust std::process::exit() use it, so exiting from a sibling
+       thread ends the process rather than only that thread. */
+    static constexpr uint64_t SYS_EXIT_GROUP           = 199;  // (exitCode) -> does not return
+
     /* Kernel-owned process capabilities. User identities may namespace
        per-user resources, but never participate in authorization decisions. */
     static constexpr uint64_t CAP_PROCESS_ADMIN = 1ULL << 0;