Fix suseconds_t type signedness for Linux ABI compatibility

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## Summary

The `suseconds_t` type is defined as unsigned (`u32`/`u64`) but should be signed (`i32`/`i64`) per Linux kernel ABI.

## Location

`litebox_common_linux/src/lib.rs` lines 771-781:

```rust
cfg_if::cfg_if! {
if #[cfg(all(target_arch = "x86"))] {
pub type time_t = i32;
pub type suseconds_t = u32; // <-- Should be i32
} else if #[cfg(all(target_arch = "x86_64"))] {
pub type time_t = i64;
pub type suseconds_t = u64; // <-- Should be i64
} else {
compile_error!("Unsupported architecture");
}
}
```

## Expected Behavior

Per Linux kernel (`include/uapi/asm-generic/posix_types.h`):

```c
typedef __kernel_long_t __kernel_suseconds_t;
```

Where `__kernel_long_t` is `long` (signed). So:
- On x86 (32-bit): `suseconds_t` should be `i32`
- On x86_64 (64-bit): `suseconds_t` should be `i64`

## Impact

This affects the `TimeVal` struct which is used in:
- `Rusage.ru_utime` and `Rusage.ru_stime` (getrusage syscall)
- `ItimerVal` (setitimer/getitimer syscalls)
- Any other syscalls using `TimeVal`

**Current impact is minimal** since:
1. `suseconds_t` values are typically non-negative microseconds (0-999999)
2. Current implementations return zeroed values

However, for strict ABI compatibility and future implementations that populate real time values, this should be fixed.

## Suggested Fix

```rust
cfg_if::cfg_if! {
if #[cfg(all(target_arch = "x86"))] {
pub type time_t = i32;
pub type suseconds_t = i32; // Changed from u32
} else if #[cfg(all(target_arch = "x86_64"))] {
pub type time_t = i64;
pub type suseconds_t = i64; // Changed from u64
} else {
compile_error!("Unsupported architecture");
}
}
```

## Additional Consideration

Consider adding compile-time size assertions for structs that depend on these types:

```rust
#[cfg(target_arch = "x86_64")]
const _: () = assert!(core::mem::size_of::() == 16);
#[cfg(target_arch = "x86")]
const _: () = assert!(core::mem::size_of::() == 8);
```

## Found During

Code review of PR #606 (getrusage syscall implementation).

In litebox_common_linux/src/lib.rs lines 771-781, inspect the architecture-specific aliases and the TimeVal definitions and usages. Confirm the aliases use signed types matching the Linux ABI on x86 and x86_64, then check the affected Rusage and ItimerVal layouts, including the proposed size assertions if included. Done when the Rust definitions and dependent layouts are ABI-compatible.

Written by the indexing model from the issue text.