Fix the MuJoCo IK fallback in the UR5e examples; add restarts to unseeded solves

[siddhss5]

Why

#65: both examples passed the collision checker in MuJoCoIKSolver's joint_limits positional slot, so the differential-IK fallback raised TypeError on its first update. Reproduced as described.

What

• Examples: each has build_ik_solver(model, data, joints, collision, menagerie, backend) returning (solver, name), with backend in auto (SSIK if installed), ssik, or mujoco, and a --ik flag on main. The fallback passes collision_checker= by keyword; limits come from the model. The SSIK path is unchanged.
• Second finding, fixed here because the fallback was never runnable before: MuJoCoIKSolver.solve without a seed started from whatever the shared MjData was last left in and converged on only 14 of 30 top-grasp poses (23 of 30 side); with random restarts it converged on 30 and 28. Unseeded solves now try the current state and then restarts (default 3) random configurations within one turn of the limits, returning every distinct converged solution, which also gives the sampler several branches per pose. Seeded solves are unchanged, so projection keeps its seed-nearest behavior.
• Changelog updated.

Fixes #65.

Test plan

New test_examples_mujoco.py, gated on mujoco and the menagerie but not on ssik, so the non-SSIK path is exercised in an environment where SSIK is installed:

• forced backend="mujoco" returns a MuJoCoIKSolver with the collision checker attached and solves a nontrivial pose from a seed to 5 mm;

• backend="auto" returns SSIKSolver when ssik is installed;

• a bad backend name is rejected;

• ur5e_mujoco.py --no-viz --ik mujoco runs end to end and reports the MuJoCo solver and a found path;

• one planning query in tsr_union_demo through the fallback succeeds with a collision-free path.

• Full suite without the menagerie: 303 passed, 2 skipped (the two menagerie-gated modules). With MUJOCO_MENAGERIE_PATH: the 7 UR5e SSIK integration tests and the 5 fallback tests pass (12 passed, about 25 s). Ruff clean.

🤖 Generated with Claude Code

[siddhss5]

Post-merge audit confirmed that #65 and #66 are fixed, then found two follow-ups associated with the expanded restart path and the combined real-model test gate:

• Make MuJoCo IK restarts topology-aware and reproducible #69: make MuJoCo IK restarts topology-aware and reproducible. The current blanket intersection with [-pi, pi] raises ValueError: high - low < 0 for a valid finite interval such as [4, 5], applies a revolute heuristic to prismatic joints, and is not seeded by the examples' --seed option.
• Make the SSIK and MuJoCo integration tests isolation-safe #70: remove order dependence from the SSIK/MuJoCo tests. test_ssik_backend.py reloads the production backend and changes the SSIKSolver class identity cached by the example modules. The focused modules pass independently, while the combined Menagerie run gives 35 passed and 1 failed at the exact-type assertion.

The original regressions remain green independently: the forced MuJoCo fallback module passes 5 tests and the transform-ownership module passes 24 tests.