feat(syscall): added faccessat and access, stubbed renameat

[FlareCoding]

Summary

• Editors probe access(W_OK) before writing, and without the syscall every file opened read-only.
• faccessat resolves through the shared dirfd path helper, with existence satisfying all modes since no permission model exists.
• renameat returns ENOSYS on purpose so callers fall back to copy-based replacement until the VFS grows a rename op.

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