Raw system calls are allowed only in std.rt

Decision

The native runtime is Oska code (ADR 0011). It reaches the operating system through system-call intrinsics (openat, read, write and a few more) that the code generator lowers directly to machine code.

These intrinsics exist only inside one module that ships with the toolchain, std.rt. That module is compiled into every program:

• The checker rejects, with its own error code, any import of std.rt and any use of an intrinsic outside it, in user code and dependency code alike.
• scripts/check.sh checks that only std.rt uses them, in the same way the unsafe guard keeps unsafe inside oska-sys (ADR 0009).

Why

A raw system call is ambient authority: code that can call openat bypasses every capability. Keeping all raw OS access in one small, toolchain-owned module gives reviewers a single place to check, and the same rule as oska-sys.

Considered options

• Make system calls the first use of the future unsafe/FFI feature. Rejected: it would pull the whole unsafe policy and the manifest allow-list into the backend work.
• Body-less prelude functions whose system-call wrappers live inside the code generator. Rejected: the system-call surface would be hidden in Rust code, not visible as Oska.