Add missing filesystem mutation primitives to std::sys

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 #370.
- [ ] Platform-specific behavior is documented instead of silently normalized.

Start at the common std::sys::fs facade and the target-specific filesystem providers, then read the contracts tracked by #375 and represented in #378. Check the hosted tests for success and representative failure paths, and review the architecture split in #370 for RV64 Linux. Done means matching Linux and macOS primitives, auditable types and return conventions, documented platform-specific behavior, and passing integration coverage.

Written by the indexing model from the issue text.