feat: experimental cross-platform Pixi support

[bburda]

Pull Request

Summary

Add experimental Pixi support for dependency management on Linux x86_64. Pixi replaces apt install ros-* + rosdep install with a lockfile-based reproducible environment from RoboStack/conda-forge. colcon remains the build system.

Two Pixi environments: Jazzy and Humble.

macOS and Windows support deferred to follow-up issues (#267, #268) due to cross-platform build failures discovered during CI testing.

---

Issue

• closes Experimental Pixi support for dependency management #265

---

Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

---

Testing

Verified locally (Jazzy on Linux):

• pixi install -e jazzy / pixi install -e humble - both resolve
• pixi run -e jazzy build - all 7 packages build
• pixi run -e jazzy test - 1271 unit tests pass, 0 failures
• pixi run -e jazzy smoke - gateway starts and responds to health check

CI: 2 non-blocking Pixi jobs (Jazzy + Humble on ubuntu-latest) with continue-on-error: true.

Issues found and fixed during implementation:

• Added brotli dep (cpp-httplib auto-enables brotli in Pixi env)
• Added numpy dep (Humble's FindPython needs it)
• Added #include <unistd.h> for _exit() portability (conda gcc stricter)
• Fixed set -u vs COLCON_TRACE in smoke test

---

Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed

Known Limitations (documented in installation.rst)

• Linux x86_64 only - macOS (Pixi: macOS (Apple Silicon) support #267) and Windows (Pixi: Windows (x86_64) support #268) have build issues
• cpp-httplib built from source - not on conda-forge
• RoboStack package coverage - not all packages available
• No integration tests in CI - unit tests + smoke test only

[Copilot]

Pull request overview

Adds an experimental Pixi-based, lockfile-driven development environment to make ros2_medkit builds reproducible across Linux/macOS/Windows (RoboStack/conda-forge), while keeping colcon as the build system.

Changes:

• Introduces pixi.toml with Jazzy/Humble environments and platform-specific pixi run tasks (symlink install on Unix, merge install on Windows).
• Adds helper scripts for Pixi activation, cpp-httplib source install into the Pixi prefix, and a smoke test for the gateway health endpoint.
• Adds non-blocking Pixi CI jobs and updates docs/README plus repo hygiene (.gitattributes, .gitignore, pre-commit exclude).

Reviewed changes

Copilot reviewed 11 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ros2_medkit_integration_tests/demo_nodes/long_calibration_action.cpp	Adds POSIX include for _exit() used in a terminate handler (portability impact).
scripts/pixi-smoke-test.sh	Smoke test script to launch the gateway and poll /api/v1/health.
scripts/pixi-install-cpp-httplib.sh	Builds/installs cpp-httplib into $CONDA_PREFIX for Pixi envs.
scripts/pixi-activate.sh	Unix activation hook to source install/setup.bash if present.
scripts/pixi-activate.bat	Windows activation hook to call install\\setup.bat if present.
pixi.toml	Defines Pixi workspace, Jazzy/Humble features, and build/test/smoke tasks by platform.
docs/installation.rst	Documents Pixi usage, tasks, and limitations.
README.md	Adds a short “Experimental: Pixi” quickstart and link to installation docs.
.pre-commit-config.yaml	Excludes pixi.lock from large-file checks.
.gitignore	Ignores .pixi/ directory.
.github/workflows/ci.yml	Adds experimental, non-blocking Pixi CI matrix (OS × distro).
.gitattributes	Marks pixi.lock as generated for GitHub diff collapsing.

[Copilot]

Pull request overview

Adds an experimental Pixi-based (RoboStack/conda-forge) workflow to create reproducible ROS 2 Jazzy/Humble development environments on Linux x86_64, plus CI smoke/unit coverage and accompanying docs.

Changes:

• Introduce pixi.toml with Jazzy/Humble environments and Pixi tasks (build/test/smoke) plus activation/install helper scripts.
• Add a non-blocking GitHub Actions job to build + smoke test + run unit tests under Pixi.
• Document the Pixi workflow and limitations in docs/installation.rst and README.md.

Reviewed changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/ros2_medkit_integration_tests/demo_nodes/long_calibration_action.cpp	Add POSIX header for _exit() portability in stricter toolchains.
scripts/pixi-smoke-test.sh	New smoke-test script to start gateway and probe /api/v1/health.
scripts/pixi-install-cpp-httplib.sh	New helper to build/install cpp-httplib into the Pixi prefix.
scripts/pixi-activate.sh	New Pixi activation hook to source colcon overlay when present.
pixi.toml	Define Pixi workspace, Jazzy/Humble features, and build/test tasks.
docs/installation.rst	Add “Experimental: Pixi” installation and task documentation.
README.md	Add top-level Pixi quick-start snippet and link to docs/issues.
.pre-commit-config.yaml	Exclude pixi.lock from large-file pre-commit check.
.gitignore	Ignore Pixi working directory (.pixi/).
.github/workflows/ci.yml	Add non-blocking Pixi CI job (Jazzy/Humble) to build/smoke/test.
.gitattributes	Mark pixi.lock as generated to collapse GitHub diffs.