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

programs/rust/rust-toolchain.toml

@@ -1,3 +1,8 @@
+# Phase 2 Rust support: the forked rustc with the built-in
+# x86_64-unknown-montauk target and a native std. Build it in ~/dev/mtk/rust,
+# then register it with:
+#   rustup toolchain link montauk ~/dev/mtk/rust/build/host/stage1
+# The linked toolchain has no cargo of its own; rustup falls back to the
+# nightly toolchain's cargo.
 [toolchain]
-channel = "nightly"
-components = ["rust-src"]
+channel = "montauk"