Honor explicit stage in HeightmapImporter

[sylvesterkaczmarek]

Description

Ensure HeightmapImporter(stage=...) consistently authors heightmap state into the stage supplied by the caller.

The importer accepted an explicit stage and used it for most geometry, but two operations bypassed it:

• _setup_stage_properties() called the global stage utility for the up axis
• _create_ground_plane() constructed GroundPlane, whose implementation targets the globally current USD stage

This change sets the up axis directly on self._stage and authors the ground plane with PhysicsSchemaTools.addGroundPlane(self._stage, ...).

Validation

• adds an in-memory-stage regression test for stage units and up axis
• existing heightmap creation tests now assert stage-targeted calls
• ground-plane sizing/centering tests continue to cover square, wide, tall, offset, empty, and disabled cases
• production change is limited to the heightmap importer