Pixi side-by-side rollout (polish)

[TenzinPlatter]

Brings the xtypes pixi side-by-side branch up to the platform_toolbox standard.

Pixi path (the work)

• CI runners: pixi workflows now use `runs-on=${{ github.run_id }}/runner=2cpu-linux-x64`.
• `publish-pixi.yml`: added a build gate (`mise ci build --package-dir=.`) before `recipes-pr` so a non-building recipe never reaches ros-recipes; teardown bumped `@v2` -> `@v3`.
• `release-pixi.yml` -> `release-pixi.yml.disabled` (avoids confusion with the canonical deb release path).
• `pixi.toml`: channel port `8000` -> `12222`, pruned empty `host/run-dependencies` tables, added a `tests` feature/env (cmake + ctest with `XTYPES_BUILD_TESTS=ON`; xtypes is a plain-CMake header-only lib, not ament/colcon).
• `pixi.lock`: regenerated against the `12222` proxy (now pins `localhost:12222/general`).
• `.taplo.toml`: aligned verbatim with toolbox; pixi.toml is taplo-formatted.

Verification

• `pixi run -e tests test`: 131/131 tests pass.

Deb path: unchanged vs main

`git diff origin/main -- CMakeLists.txt cmake/ package.xml include/ test/ examples/ release.yml ...` is empty. No source/CMake/package.xml touched.

Risks / follow-ups

• xtypes uses the stock `pixi-build-cmake` backend (not `pixi-build-ros-gr`), so the ros-gr backend pin / ament test-dep guidance does not apply; cmake backend left at `*` (it is not pinned in the lock either).
• No `.pre-commit-config.yaml` exists in this repo (none on main); the taplo pre-commit hook was not added to avoid fabricating CI infra. pixi.toml is still taplo-formatted via `.taplo.toml`.