feat: pixi conversion

[tenzinplatter]

Polishes the pixi side-by-side path on launch_ext to match the platform_toolbox reference (PR #61). Single root-level ament_python package.

Pixi path

• Workflows: pixi runners -> `runs-on=${{ github.run_id }}/runner=2cpu-linux-x64`; `publish-pixi.yml` gains a `mise ci build --package-dir=.` gate before `recipes-pr`; teardown bumped to @V3. `release-pixi.yml` renamed to `release-pixi.yml.disabled`.
• `pixi.toml`: channel port 8000 -> 12222 (`/general`), backend pinned `pixi-build-ros-gr >=0.4,<0.5`, build-number already inside `[package.build.config]`. Added `[feature.tests]` env (colcon build+test) and `[environments] tests`. Dropped unused `hatchling` host-dep.
• Removed `pyproject.toml`: it was a stray hatchling manifest that broke the colcon source build (setuptools tried to apply its `[project]` table and crashed). The pixi-build-ros-gr backend builds ament_python from `setup.py` (same as toolbox), so it is unnecessary.
• `.taplo.toml`: added `[formatting]` block; pixi.toml reformatted by taplo. Added taplo-format pre-commit hook (union with existing hooks).
• `pixi.lock` regenerated against the 12222 proxy.

Deb path: unchanged vs main

No deb-path file differs from `origin/main` (package.xml, setup.py, sources, tests, release.yml, ci.yml.disabled, devcontainer all byte-identical).

Verification

• Backend build (publish gate): `pixi build` -> "Successfully published 1 package(s)" (launch-ext-1.22.3).
• colcon tests env: 22/23 pass. The 1 failure (`test_include_package_launch_file`) asserts a hardcoded `/opt/ros/kilted/...` path; it is environment-specific to the deb install layout, not a pixi defect.

Risks

• Large `pixi.lock` churn from the port bump (expected).
• The one path-assertion test failure is pre-existing/environment-specific.