fix: give SceneNode.__init__ a pose= kwarg, not just Shape

[petercorke]

Summary

SceneNode is the true base class for everything in the scene graph, but
the friendly pose: ndarray | SE3 constructor kwarg (with SE3 support)
was only ever implemented on Shape.__init__, one level below it.

• SceneGroup subclasses SceneNode directly (not Shape), so
  SceneGroup(pose=SE3(...)) raised
  TypeError: SceneNode.__init__() got an unexpected keyword argument 'pose'
  -- only the raw T=ndarray kwarg worked.
• CollisionShapeGroup happened to accept pose= already, but only as a
  side effect of going through CollisionShape/Shape for unrelated
  reasons (it needs to be a CollisionShape for isinstance checks
  elsewhere), not because pose was deliberately designed to work there.

This moves pose (and its SE3 -> ndarray conversion) down to
SceneNode.__init__ itself, so every scene-graph node -- including groups
-- gets it consistently and for the right reason. Shape.__init__'s own
base/pose deprecation logic is unchanged; it now just forwards its
resolved value up as pose= instead of T=.

• SceneNode.__init__: T renamed to pose: ndarray | SE3, same
  conversion Shape.__init__ already did, plus a docstring (previously had
  none)
• SceneNode.__deepcopy__ and Shape.__init__'s super().__init__()
  call updated to the new kwarg name
• Regression test: SceneGroup(pose=...) with both SE3 and ndarray

Test plan

• pytest tests/test_Shape.py tests/test_collision.py -- 157 passed
• Full suite: pytest tests/ -- 160 passed
• Confirmed SceneGroup(pose=SE3(1,2,3)) no longer raises
• Confirmed base/pose mutual-exclusivity ValueError on Shape still fires unchanged