feat: Implement fchdir syscall

[wdcui]

Summary

This PR adds support for the fchdir(2) syscall, which changes the current working directory to the directory referred to by a file descriptor.

Changes

litebox_common_linux

• Add Fchdir { fd: i32 } variant to SyscallRequest enum
• Add Sysno::fchdir syscall parsing

litebox_shim_linux

• Add DirectoryPath metadata type for storing directory paths
• Add cwd field to FsState for per-process CWD tracking
• Implement compute_absolute_path() helper method
• Update sys_open() to store directory paths as entry metadata
• Update sys_getcwd() to return actual CWD from FsState (was returning hardcoded "/")
• Implement sys_fchdir() to change CWD based on directory fd
• Add dispatch case for Fchdir in do_syscall

Error Handling

The implementation correctly handles:

• EBADF: Invalid or negative file descriptor
• ENOTDIR: File descriptor refers to a non-directory (regular file, socket, pipe, etc.)

Tests Added

• test_fchdir_basic: Basic fchdir functionality
• test_fchdir_ebadf: Invalid fd handling (returns EBADF)
• test_fchdir_enotdir: Non-directory fd handling (returns ENOTDIR)
• test_fchdir_to_root: Changing back to root directory

Testing

All existing tests pass. New tests verify the basic functionality and error handling.

cargo test --package litebox_shim_linux test_fchdir
# 4 tests pass

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
  variant SyscallRequest::Prctl 90 -> 91 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2256
  variant SyscallRequest::Alarm 91 -> 92 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2259
  variant SyscallRequest::SetITimer 92 -> 93 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2262