translate URDF <mimic> joints to joint equality constraints

[Abhayindia]

per #3527. urdf <mimic multiplier="M" offset="O"> on a follower joint becomes an <equality><joint> with polycoef = [O M 0 0 0]. couplings are collected during joint parsing and emitted after, so the target joint can be declared later in the file.

also wired xml_urdf_test.cc into cmake - it wasn't referenced by any target so its tests weren't building.

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[Abhayindia]

added the guard - a mimic pointing at a fixed/planar/floating/missing joint now warns and is skipped rather than failing the load, with a test for the fixed-target case. thanks for catching that.

[sylvesterkaczmarek]

Could we make the mimic regression behavioral rather than only checking the emitted IDs and polycoef? The current test would still pass if the equality were wired in the wrong direction or interpreted incorrectly relative to the joint references. A small case with a nonzero driver value, multiplier=-1, and nonzero offset that checks the resulting equality residual/tracking would pin the actual URDF q_follower = M*q_driver + O semantics.

[Abhayindia]

done - MimicBecomesJointEquality now simulates from a driver displacement (multiplier=-1, offset=0.02, damping so it settles) and checks q_right == -q_left + 0.02 to 1e-4, so it'd catch a wrong direction or a dropped multiplier/offset.