Derive NewtonActuator from UsdTyped instead of UsdGeomImageable

Per the schema working group decision, actuators are not imageable. An
actuator carries a control law bound to a joint via `newton:targets`; it
has no extent and is never rendered.

This also fixes the crash reported in #77. `plugInfo.json` declared
`UsdGeomImageable` as the base, but the prim definition in
`generatedSchema.usda` only contains `newton:targets` — the inherited
`visibility`, `purpose` and `proxyPrim` properties were never baked in.
`UsdGeom.BBoxCache` therefore recognized the prim as Imageable and then
failed to read its visibility attribute:

    Failed verification: ' attr ' -- Unable to get attribute
    'Visibility' on prim at path </World/Actuator>

`UsdTyped` contributes no properties, so the declared base and the prim
definition are now consistent with no change to `generatedSchema.usda`.
This matches how core USD types the same category of non-renderable
prim: `PhysicsScene`, `PhysicsCollisionGroup`, `UsdShadeShader`,
`UsdRenderVar` and `UsdSkelAnimation` are all concreteTyped on UsdTyped.

Note that `UsdGeomScope` would not have fixed the crash, as it is also
Imageable.

Author: andrewkaufman

newton_usd_schemas/plugInfo.json

@@ -175,7 +175,7 @@
               },
               "autoGenerated": false,
               "bases": [
-                "UsdGeomImageable"
+                "UsdTyped"
               ],
               "schemaKind": "concreteTyped"
             },

tests/test_actuator.py

@@ -4,7 +4,7 @@
 import math
 import unittest
 
-from pxr import Plug, Sdf, Usd
+from pxr import Plug, Sdf, Usd, UsdGeom
 
 import newton_usd_schemas  # noqa: F401
 
@@ -35,6 +35,27 @@ def test_targets_relationship(self):
         self.assertEqual(len(targets), 1)
         self.assertEqual(str(targets[0]), "/World/Joint")
 
+    def test_not_imageable(self):
+        # Actuators carry a control law bound to a joint; they have no extent and are
+        # never rendered, so they derive from UsdTyped rather than UsdGeomImageable.
+        self.assertFalse(self.prim.IsA(UsdGeom.Imageable))
+
+        # The prim definition must not advertise Imageable properties it does not have.
+        definition = Usd.SchemaRegistry().FindConcretePrimDefinition("NewtonActuator")
+        self.assertIsNotNone(definition)
+        self.assertEqual(sorted(definition.GetPropertyNames()), ["newton:targets"])
+
+    def test_bbox_cache_skips_actuator(self):
+        # Regression test: when NewtonActuator declared UsdGeomImageable as its base
+        # without baking in the inherited visibility/purpose/proxyPrim properties,
+        # UsdGeom.BBoxCache treated the prim as Imageable and then failed to read its
+        # visibility attribute, raising from _GetOrCreateVisibilityQuery.
+        root = self.stage.DefinePrim("/World", "Xform")
+        self.stage.DefinePrim("/World/Actuator", "NewtonActuator")
+        cache = UsdGeom.BBoxCache(Usd.TimeCode.Default(), [UsdGeom.Tokens.default_])
+        bound = cache.ComputeWorldBound(root)
+        self.assertTrue(bound.GetRange().IsEmpty())
+
 
 class TestNewtonActuatorDelayAPI(unittest.TestCase):
     def setUp(self):