feat: add Windows build support
Add conditional compilation for Unix-specific features to enable Windows builds:
- Wrap FIFO-related code with cfg(unix) attributes
- Use platform-specific exit status handling in cargo-codspeed
- Make fifo module Unix-only in lib.rs
- Guard FIFO constants and operations across all crates
This allows the project to build on Windows (build-only, not runtime support).
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Co-Authored-By: Claude <noreply@anthropic.com>