Add cutout shell and volume node assets for depth volume processing

- Implemented `cutout_shell.py` to build front normals and shell positions for balloon geometry.
- Created `cutout_volume.py` to establish smooth center fields and manage balloon surface attributes.
- Introduced `pisa_sr_export.py` for exporting PiSA-SR components with dependencies and asset finalization.

tests/test_node_assets.py

@@ -24,7 +24,7 @@ class NodeAssetsTest(unittest.TestCase):
             for group in target.node_groups
             if group.bl_idname == "GeometryNodeTree"
         ]
-        self.assertEqual(len(groups), 7)
+        self.assertEqual(len(groups), 5)
         for group in groups:
             validate_node_layout(group)