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.

tools/sync_models.py

@@ -193,6 +193,14 @@ def prepare_model(model):
     if model.build:
         build_model(model)
         return
+    if model.key == "PISA_SR":
+        subprocess.run(
+            ["uv", "run", "--script", str(PROJECT_ROOT / "tools/pisa_sr_export.py")],
+            cwd=PROJECT_ROOT, check=True,
+        )
+        if not verify_file(model.destination, model.size, model.sha256):
+            raise RuntimeError("PiSA-SR export differs from the published catalog")
+        return
     if not model.source_url:
         raise RuntimeError(f"No upstream source or builder for {model.key}")
     download_file(