Pixi: Windows (x86_64) support

[bburda]

Summary

Add Windows (win-64) platform support to the experimental Pixi environment.

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Proposed solution

Currently blocked by build failures in ros2_medkit_serialization with MSVC. CI logs were truncated but the failure occurs during compilation. Additional considerations:

• --merge-install needed instead of --symlink-install (symlinks require admin on Windows)
• BUILD_TESTING=OFF needed (ament_cmake_clang_format/tidy unavailable on win-64)
• Smoke test script uses bash process backgrounding which may not work in Git Bash
• pixi-activate.bat was prepared but removed when scoping to Linux-only

Both Jazzy and Humble are affected.

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Additional context

• Parent issue: Experimental Pixi support for dependency management #265
• PR with Linux-only Pixi support: feat: experimental cross-platform Pixi support #266
• CI logs: https://github.com/selfpatch/ros2_medkit/actions/runs/23092404160
• RoboStack provides Jazzy/Humble packages for win-64