fix: guard mouse capture bytes assertion for Windows

[noahgift]

Summary

• Guards the bytes-written assertion in test_testable_backend_enable_mouse_capture with #[cfg(not(target_os = "windows"))]
• crossterm uses Win32 Console API on Windows instead of ANSI escapes, so no bytes are written to the Vec<u8> writer
• The is_mouse_captured() assertion already verifies correctness on all platforms
• Also applies cargo fmt to pre-existing formatting issues in build.rs and contract_traits.rs

Closes #37

Five-Whys

1. output Vec is empty after enable_mouse_capture() on Windows
2. crossterm uses Windows Console API, not ANSI escape sequences
3. EnableMouseCapture dispatches to SetConsoleMode on Windows
4. Test assumes ANSI output (Unix behavior)
5. Main CI runs on Linux; cross-platform added later

Test plan

• Cross-platform CI passes on Windows
• Main CI still passes on Linux

🤖 Generated with Claude Code

[github-actions]

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