Implement symlink and symlinkat syscalls

[wdcui]

Summary

This PR adds support for the symlink(2) and symlinkat(2) Linux syscalls.

Changes

Syscall Parsing (litebox_common_linux)

• Added Symlinkat variant to SyscallRequest enum
• Added syscall number parsing for Sysno::symlink and Sysno::symlinkat
• Added From<SymlinkError> and From<ReadlinkError> for Errno conversions

Filesystem Layer (litebox/src/fs)

• Added SymlinkError and ReadlinkError error types in errors.rs
• Added FileType::SymbolicLink variant to FileType enum
• Added symlink() and readlink() methods to FileSystem trait
• Implemented in all filesystems:
  • in_mem.rs: Full implementation with Entry::Symlink and SymlinkX types
  • layered.rs: Delegates to upper/lower layers appropriately
  • tar_ro.rs: Returns ReadOnlyFileSystem for symlink, simplified readlink
  • devices.rs: Returns error (devices don't support symlinks)

Syscall Handler (litebox_shim_linux)

• Added sys_symlink() and sys_symlinkat() methods to Task
• Updated do_readlink() to check filesystem for symlinks
• Added SyscallRequest::Symlinkat dispatch case in main handler

Testing

• Added 7 unit tests for symlink functionality
• All 182 local tests pass

Design Notes

Following POSIX/Linux semantics, symlink targets do NOT need to exist at creation time. The target path is validated only when the symlink is followed, not when it's created.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_marked_non_exhaustive: enum marked #[non_exhaustive] ---

Description:
A public enum has been marked #[non_exhaustive]. Pattern-matching on it outside of its crate must now include a wildcard pattern like `_`, or it will fail to compile.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#attr-adding-non-exhaustive
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/enum_marked_non_exhaustive.ron

Failed in:
  enum TruncateError in /home/runner/work/litebox/litebox/litebox/src/fs/errors.rs:78