_selfhost_compile_42.nx

_selfhost_compile_42.nx -- end-to-end self-host validation. Compiles NishiLang source `func main() -> i64 { return 42 }` to x86_64 assembly using the NishiLang-in-NishiLang pipeline (nx_nlex + nx_nparse + nx_nir + nx_nx86), then writes the asm bytes to stdout (fd=1) via sys_write. The bench harness captures stdout, runs it through gas + ld, executes the resulting ELF, and verifies the exit code is 42. This closes the validation loop: our NishiLang compiler produces an actually-running binary.