Drop the distro mutex's run_constraints

[Tobias-Fischer]

As discussed with @traversaro: the mutex only keeps one ROS distribution per environment. Library versions come from the packages' own dependencies, and wrong metadata is fixed in the package itself (a patch, pkg_additional_info.yaml, or a conda-forge repodata patch), not in the mutex.

Why: mutex run_constraints apply to every user environment, also to libraries the user's packages never use. When another package needs a different version, solvers silently fall back to old ROS builds. For example, ros-base + libprotobuf 6.*:

	with constraints	without
jazzy	mutex 0.15, rclcpp 28.1.18	mutex 0.19, rclcpp 28.1.22
lyrical	mutex 0.18, rclcpp 32.0.0	mutex 0.21, rclcpp 32.0.3

Verification: I solved desktop + perception + ros_gz for every distribution on linux-64, osx-arm64 and win-64, with the published mutex and with an unconstrained copy of it:

• humble, lyrical: identical environments. The shared libboost/pcl/libprotobuf/vtk pins and humble's krb5 were redundant, since the packages already pin these themselves.
• jazzy, rolling: newer builds (openvino 2026.4, ffmpeg 9.0.2, newer OpenCV builds). The constraints added to work around solver trouble only held these back now. I installed the jazzy environment on osx-arm64 and ran: OpenCV 5 with dnn, cv_bridge, rclpy, C++ talker → listener, Gazebo Harmonic, and gz-msgs/gz-transport with libprotobuf 7.35.1.

Changes:

• shared/vinca.yaml: run_constraints: [] (vinca requires the key) and a comment stating the policy. The per-distribution constraints are removed.
• Each distribution publishes a new build of its mutex at the same version (mutex_package.build_number = build_number + 1), so no other package needs a rebuild. This needs Mutex: honour mutex_package.build_number; run_constraints optional vinca#169: vinca never generated such a build before. vinca is pinned to f9d68bb on robostack-integration. With that pin, lyrical's mutex recipe is number: 28, string: lyrical_28, run_constraints: [].
• rs check rejects mutex run_constraints, so they can't come back as a quick fix.
• Pinning bot: the mutex constraint sync and the mutex rebuild are gone.
• Docs: README and the robostack-versions skill.
• First unit tests of the tooling: pixi run test (pytest, tools/tests), run in the PR check job.
  • They cover the guard, every distribution's assembled config, and the partial rebuild leaving the mutex alone.
  • Re-adding a constraint makes them fail.

Follow-up: check_dependency_compat.py still has its mutex-constraint code (PIN MISMATCH, --mutex-only). With an empty list it is a no-op; I'll remove it separately.

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