Add missing filesystem mutation primitives to std::sys

[LunaStev]

Tracked by #375 and represented in #378.

Scope

Extend target-specific filesystem providers and the common std::sys::fs facade with coherent contracts for:

• rename.
• chmod and fchmod.
• truncate and ftruncate.
• link and symlink.
• readlink.
• directory creation and removal parity where names differ by platform.

Keep these as raw operating-system primitives. User-facing naming and ownership decisions belong to #380.

Completion criteria

• Linux and macOS providers expose matching portable operations where semantics align.
• Constants, integer widths, pointer types, and return conventions are source-auditable.
• Hosted tests cover success and representative failure paths.
• RV64 Linux integration follows the architecture split in Port the Linux standard-library syscall layer to RV64 #370.
• Platform-specific behavior is documented instead of silently normalized.