fs.nx

fs.nx -- file I/O stdlib (Phase G1 in the roadmap). Typed wrappers around Linux / NishiOS file syscalls. Every fallible operation returns `*Result<i64, FsError>`. Callers match on the variant -- the `-errno` sentinel convention is gone from the public API. Raw-errno helpers remain as `fs_*_raw` for the rare case where a caller wants the bare syscall return. Syscall numbers (Linux RV64, NishiOS matches): 56 openat 57 close 63 read 64 write 80 fstat 34 mkdirat 82 renameat AT_FDCWD = -100 is the "current working directory" magic fd used by the *at syscalls when the path is relative.