[BUG FIX] Fix joint-equality reindexing when attaching a floating-base entity.

[duburcqa]

Description

RigidEntity.attach() drops the floating base joint of the child entity and shifts the dof_start / q_start of all following joints, but it left joint._idx and the joint-equality references (eq_obj1id / eq_obj2id) untouched. Since joints_info is built densely by enumeration, those stale absolute indices end up off by n_base_joints, so a mimic (joint-equality) constraint of the attached entity reads the wrong dof_start -- aliasing the free-joint DOFs of an unrelated entity and dragging it around (or reading out of bounds).

Fix: shift joint._idx and the JOINT-type equality references in lockstep with the existing dof_start / q_start shift. Link-based equalities (connect/weld) are unaffected since attach() removes no link.

Related Issue

Resolves Kashu7100/Eden#589.

Motivation and Context

Attaching a floating-base entity that carries mimic joints (e.g. a dexterous hand) silently corrupted the dynamics of unrelated free bodies in the scene.

How Has This Been / Can This Be Tested?

Strengthened test_merge_entities: the unrelated free box must keep its lateral position while the attached gripper actuates. On main it drifts ~0.04 m in x; with the fix it stays put. Fails for the is_fixed=False variants without the fix, passes for all four with it.

Checklist:

• I read the CONTRIBUTING document.
• I followed the Submitting Code Changes section of CONTRIBUTING document.
• I tagged the title correctly (including BUG FIX/FEATURE/MISC/BREAKING)
• I updated the documentation accordingly or no change is needed.
• I tested my changes and added instructions on how to test it for reviewers.
• I have added tests to cover my changes.
• All new and existing tests passed.

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