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.

openspec/changes/reorganize-cutout-shapes/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-09-07