Add example of trajectory blending

[marioprats]

Example for https://github.com/PickNikRobotics/moveit_pro/pull/17705

[davetcoleman]

[written by AI]

Stale PR sweep. Opened 2026-03-26, last commit 2026-03-26. We are cleaning up open PRs older than three months.

State: one file, src/lab_sim/objectives/blend_joint_trajectories.xml, which is not on main, so the example never landed. No reviews and no comments in five months. BlendJointTrajectories still exists in the product, so the example is not obsolete, just unreviewed.

Recommendation: finish it. Confirm the objective still loads against current lab_sim, mark it ready, and merge. A worked example of trajectory blending is the kind of thing customers look for and it costs one file.

@marioprats do you still want this in? If not we will close it.