feat: add Rust userspace support; fix TLS layout for p_align < 16; kill whole process on sibling-thread faults

programs/rust/README.md

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+# Rust on MontaukOS
+
+Phase 1: `no_std` + `alloc` programs, cross-compiled from Linux with a
+stock nightly toolchain and a custom target spec. No compiler fork.
+
+## Setup (once)
+
+```bash
+curl --proto '=https' -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain none
+~/.cargo/bin/rustup toolchain install nightly --component rust-src
+```
+
+`rust-toolchain.toml` selects nightly + `rust-src` for this directory.
+`make programs` builds the Rust programs when that toolchain is present
+and prints a notice and skips them otherwise. It never downloads a toolchain
+(`RUSTUP_AUTO_INSTALL=0`).
+
+## Building
+
+```bash
+make -C programs rust          # cargo build --release, copies to bin/os/*.elf
+cd programs/rust && cargo build --release
+```
+
+Output: `target/x86_64-unknown-montauk/release/<name>`, installed as
+`programs/bin/os/<name>.elf`. Add new binaries to `RUST_PROGRAMS` in
+`programs/GNUmakefile` and to the workspace `members`.
+
+## Layout
+
+| Path | Purpose |
+|------|---------|
+| `x86_64-unknown-montauk.json` | Target spec: static non-PIE ET_EXEC, no red zone, local-exec TLS, panic=abort |
+| `.cargo/config.toml` | `-Zbuild-std=core,alloc`, linker = `x86_64-montauk-gcc` |
+| `montauk-rt/` | Runtime: syscalls, allocator, `print!`, argv, threads, futex `Mutex`, panic handler |
+| `rusthello/` | Smoke test (`rusthello`, `rusthello panic`, `rusthello panic-thread`) |
+
+The GCC driver links crt1/crti/crtbegin, the libc and libgcc from
+`toolchain/sysroot`, the same as ports. After changing `programs/lib/libc`,
+re-run `toolchain/build-montauk-toolchain.sh` so Rust programs pick it up.
+
+Syscall numbers and status codes are generated at build time by
+`montauk-rt/build.rs` from `programs/include/Api/Syscall.hpp` and
+`kernel/src/Common/StatusCodes.hpp`, so there is nothing to keep in sync by hand.
+
+## Writing a program
+
+```rust
+#![no_std]
+#![no_main]
+extern crate alloc;
+
+montauk_rt::entry!(main);
+
+fn main() -> i32 {
+    montauk_rt::println!("hello from Rust");
+    0
+}
+```
+
+## Limits (Phase 1)
+
+- No `std`. No `std::fs`, `std::net`, `std::process` or `std::thread`; use
+  `montauk_rt` equivalents where they exist.
+- `panic = "abort"`. A panic on any thread prints the message and kills the
+  whole process.
+- Thread stacks are owned by the `JoinHandle`. An unjoined thread leaks its stack.
+
+Phase 2 (a forked rustc at `~/dev/mtk/rust` with a native `target_os =
+"montauk"` std) replaces the JSON spec with a built-in target.