fix(compat): terminate itoa output reliably

[arazmj]

Summary

• replace the Unix itoa implementation's non-portable std::stringbuf::pubsetbuf use with direct conversion
• preserve base 2-36 conversion and signed decimal behavior, including INT_MIN
• guarantee that the caller's output buffer is populated and null-terminated

Multiplayer impact

The previous libc++ implementation did not reliably write into the caller-provided buffer. Multi-exit transports therefore built corrupted logical bone names such as ExitStart0..., fell back to the transport position, and desynchronized macOS from Windows while unloading passengers.

With this change, replaying the reported Infantry General Troop Crawler unload changes the macOS frame-7400 CRC from 9C0ABBE5 to the exact Windows CRC F9A12D0B.

Validation

• built GeneralsX and GeneralsXZH with the macos-vulkan preset
• verified decimal, negative, INT_MIN, hexadecimal, and invalid-base conversions
• replayed the captured mixed-platform match through frame 7400