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

programs/rust/montauk-rt/src/lib.rs

@@ -1,9 +1,9 @@
 //! Minimal `no_std` runtime for Rust programs on MontaukOS.
 //!
-//! Phase 1 of Rust support: `core` + `alloc` on a stock nightly with a
-//! custom target spec. Programs link against the Montauk libc through the
-//! x86_64-montauk GCC driver, so crt1.o owns `_start` (argv, environment,
-//! .init_array) and calls the `main` that [`entry!`] defines.
+//! For programs that use `core` + `alloc` without `std` (most programs can use
+//! `std` directly; see ../README.md). Programs link against the Montauk libc
+//! through the x86_64-montauk GCC driver, so crt1.o owns `_start` (argv,
+//! environment, .init_array) and calls the `main` that [`entry!`] defines.
 //!
 //! ```ignore
 //! #![no_std]