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_model_catalog.py

@@ -81,7 +81,7 @@ class ModelCatalogTest(unittest.TestCase):
             "SAFMN_REAL_X4": ("safmn-real-x4.onnx", 22657477, "8f13867b"),
             "REALESRGAN_X4PLUS": ("realesrgan-x4plus.onnx", 67051618, "4ed6a45a"),
         }
-        self.assertEqual(set(self.models.UPSCALE_MODELS), set(expected))
+        self.assertEqual(set(self.models.UPSCALE_MODELS), set(expected) | {"PISA_SR"})
         for key, (filename, size, checksum_prefix) in expected.items():
             with self.subTest(model=key):
                 model = self.models.get_downloadable_model(key)