Pixi: macOS (Apple Silicon) support

[bburda]

Summary

Add macOS (osx-arm64) platform support to the experimental Pixi environment.

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Proposed solution

Currently blocked by a build failure in ros2_medkit_serialization (vendored dynmsg):

yaml-cpp/node/impl.h:227:14: error: implicit instantiation of undefined template
  'YAML::convert<std::__bit_const_reference<std::vector<bool>>>'

This is a yaml-cpp / Apple Clang incompatibility with std::vector<bool> reference handling in message_reading_cpp.cpp.

Possible fixes:

• Patch vendored dynmsg to avoid vector<bool> iteration pattern
• Use a yaml-cpp version that handles this (if one exists)
• Add a workaround specific to Apple Clang

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 osx-arm64