Fix sibling URDF mimic multiplier relation

[moonbow721]

Summary

Fix URDF <mimic> constraints for sibling joints with a non-unit multiplier or non-zero offset.

SAPIEN loads URDF <mimic> joints as fixed tendons (python/py_package/wrapper/articulation_builder.py). The two topology branches order their tendon links differently:

• chain (joint mimics parent): [grandparent, source_child, follower_child]
• sibling (2 children mimic each other): [root, follower_child, source_child]

but both reuse the same coefficient vector [0, -multiplier, 1]. Because the source-child and follower-child links sit in opposite positions, the -multiplier coefficient lands on the source in the chain case (correct) but on the follower in the sibling case (wrong). The sibling constraint therefore becomes

q_source = multiplier * q_follower + offset
# i.e.  q_follower = (q_source - offset) / multiplier    <- inverted

instead of the intended URDF relation

q_follower = multiplier * q_source + offset

For the common multiplier=1, offset=0 case this is invisible, which is why it went unnoticed; but any non-unit ratio is inverted, and any non-zero offset is applied to the wrong joint. This PR changes the sibling coefficients to [0, 1, -multiplier] and the reciprocals to [0, 1, -1 / multiplier], matching the URDF convention. The chain branch is already correct and is left unchanged.

Test

Adds test_urdf_mimic_sibling_multiplier_offset: a minimal sibling-joint URDF with

<mimic joint="source" multiplier="2.0" offset="0.1"/>

drives source and asserts q_follower ~= 2.0 * q_source + 0.1. This fails on the current code (the follower instead satisfies the inverted (q_source - 0.1) / 2) and passes with the fix.

Validation

Loaded the sibling URDF on a SAPIEN 3.0.3 wheel (drive source to 0.4, follower left free):

build	source	follower	satisfies
before (current)	0.3998	0.1499	(source - 0.1) / 2 (inverted)
after (this PR)	0.3999	0.8998	2 * source + 0.1 (URDF-correct)

The chain topology satisfies 2 * source + 0.1 both before and after, confirming the fix is scoped to the sibling branch only.

Note: the repo's GitHub workflow builds wheels but does not run unittest/, so the added regression test was run locally.

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