Experimental Pixi support for dependency management

[bburda]

Summary

Add experimental Pixi (prefix.dev) support as an alternative dependency management path for ros2_medkit. Pixi uses conda-forge/robostack packages to provide reproducible, lockfile-based environments without requiring a system-wide ROS 2 installation.

This is not intended to replace the standard ROS 2 toolchain - it's an experimental path to evaluate feasibility and developer experience.

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Proposed solution

Single PR covering:

1. Pixi setup - pixi.toml with Jazzy and Humble environments (linux-64)
2. CI jobs - 2 non-blocking GitHub Actions jobs (Jazzy + Humble on ubuntu-latest)
3. Documentation - docs page + README note marking Pixi as experimental

Cross-platform status

• Linux x86_64: working (PR feat: experimental cross-platform Pixi support #266)
• macOS (Apple Silicon): blocked by dynmsg/yaml-cpp build issue (Pixi: macOS (Apple Silicon) support #267)
• Windows: blocked by MSVC build issue (Pixi: Windows (x86_64) support #268)

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Additional context

• Pixi: https://pixi.sh
• Robostack (ROS on conda-forge): https://robostack.github.io
• colcon still needed for building - Pixi manages the environment/deps layer