feat: implement setuid/setgid syscall family

[wdcui]

Summary

Implement 8 syscalls for managing user and group IDs:

• setuid, setgid: Set all three IDs (real, effective, saved)
• setreuid, setregid: Set real and effective IDs
• setresuid, setresgid: Set real, effective, and saved IDs independently
• getresuid, getresgid: Get all three IDs

Motivation

These syscalls are commonly used by applications that need to drop privileges for security. For example, a server might start as root and then drop to a non-privileged user.

Changes

Credentials Struct

• Extended Credentials struct with suid (saved-set-user-ID) and sgid (saved-set-group-ID) fields
• Changed storage from Arc<Credentials> to RefCell<Credentials> to allow mutation

Syscall Implementation

• Added SyscallRequest variants and parsing for all 8 syscalls
• Implemented handlers with -1 (no-change) semantics per POSIX
• All syscalls always succeed (no permission checks) since litebox is a sandboxed single-process environment

Testing

• Added 6 unit tests covering all syscalls
• Tests verify -1 no-op behavior, partial updates, and correct state tracking

Files Modified

• litebox_common_linux/src/lib.rs - Added SyscallRequest variants and parsing
• litebox_shim_linux/src/lib.rs - Changed credentials to RefCell, added dispatch
• litebox_shim_linux/src/syscalls/process.rs - Updated Credentials struct, added handlers
• litebox_shim_linux/src/loader/auxv.rs - Fixed credentials access

Testing Done

• All 6 new unit tests pass
• Full test suite: 161/176 tests pass (same 15 pre-existing failures as main branch, all TUN/Node.js related)

[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::Sysinfo 82 -> 90 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2266
  variant SyscallRequest::CapGet 83 -> 91 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2269
  variant SyscallRequest::GetDirent64 84 -> 92 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2273
  variant SyscallRequest::SchedGetAffinity 85 -> 93 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2278
  variant SyscallRequest::SchedYield 86 -> 94 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2283
  variant SyscallRequest::Futex 87 -> 95 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2284
  variant SyscallRequest::Execve 88 -> 96 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2287
  variant SyscallRequest::Umask 89 -> 97 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2292
  variant SyscallRequest::Prctl 90 -> 98 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2295
  variant SyscallRequest::Alarm 91 -> 99 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2298
  variant SyscallRequest::SetITimer 92 -> 100 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2301