DHRobot tests fail against SG's new scene-graph cycle detection when sharing link objects across robots

New finding, 2026-08-09, while getting the test suite green again after unvendoring spatialgeometry (see the fix/unvendor-spatialgeometry PR) and bumping the effective spatialgeometry API to SG's current main.

tests/test_DHRobot.py::TestDHRobot::test_isspherical fails against SG's main:

ValueError: Cannot set RevoluteDH(...)'s scene_parent to RevoluteDH(...) --
RevoluteDH(...) is already a descendant of RevoluteDH(...), this would
create a cycle in the scene graph

The test builds several DHRobots that reuse the same RevoluteDH link instances in different combinations/orderings:

l0 = rp.RevoluteDH()
l1 = rp.RevoluteDH(alpha=-np.pi / 2)
l2 = rp.RevoluteDH(alpha=np.pi / 2)
l3 = rp.RevoluteDH()

r0 = rp.DHRobot([l0, l1, l2, l3])
r1 = rp.DHRobot([l0, l1])
r2 = rp.DHRobot([l1, l2, l3, l0])   # <-- fails here

Each DHRobot.__init__ sets link.scene_parent = link.parent for every link (BaseRobot._sort_links). SG recently added cycle-detection to scene_parent's setter (a real safety feature -- an undetected cycle in the scene graph previously caused update()'s root-finding walk to spin forever). Because l0/l1/l2/l3 are shared objects, r0's construction already wired l1.scene_parent = l0 (from the [l0,l1,l2,l3] chain); when r2 later tries l0.scene_parent = l3 in a different order, SG correctly notices l0 is already an ancestor of l3 from r0's wiring and refuses, since these are the same link objects, not fresh copies.

Open question, not yet investigated: is this DHRobot pattern (constructing multiple robots from overlapping/reordered sets of shared link objects) supposed to be supported? If so, DHRobot/BaseRobot needs to stop treating scene_parent as if each link belongs to exactly one robot's scene graph (possibly relevant to the Robot/Link kinematic-vs-scene-graph-state decoupling work tracked elsewhere). If not -- i.e. link objects were never meant to be shared across multiple DHRobot instances -- the fix is on the test side (construct fresh link instances per robot) and the test itself is what should change, not the library.

Start with tests/test_DHRobot.py::TestDHRobot::test_isspherical and the DHRobot.init path through BaseRobot._sort_links. Determine whether overlapping, reordered shared links across DHRobot instances are supported, using the scene_parent cycle error and the referenced unvendor-spatialgeometry change as context. Done means a documented decision followed by either a library fix or updated test setup.