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/node_assets/cutout_layout.py

@@ -0,0 +1,53 @@
+"""Place independent Cutout stages using authored local layouts."""
+from collections import Counter
+
+
+def route_geometry(group, source, positions):
+    """Guide a shared forward geometry reference through authored waypoints."""
+    for position in positions:
+        reroute = group.nodes.new("NodeReroute")
+        reroute.location = position
+        group.links.new(source, reroute.inputs[0])
+        source = reroute.outputs[0]
+    return source
+
+
+def layout_stage(group, previous, positions, input_regions, offset=(0, 0)):
+    """Map stage-local default names without renaming Blender nodes."""
+    counts, stage = Counter(), {}
+    for node in group.nodes:
+        if node in previous or node.bl_idname == "NodeGroupInput":
+            continue
+        base, dot, suffix = node.name.rpartition(".")
+        base = base if dot and suffix.isdigit() else node.name
+        index = counts[base]
+        counts[base] += 1
+        stage[base if index == 0 else f"{base}.{index:03d}"] = node
+    assert set(stage) == set(positions) - {"Set Shade Smooth", "Group Output"}, (group.name, set(stage) ^ set(positions))
+    for name, node in stage.items():
+        x, y = positions[name]
+        node.location = (x + offset[0], y + offset[1])
+    inputs = [(link.to_socket, link.from_socket.name) for link in group.links if link.to_node in stage.values() and link.from_node.bl_idname == "NodeGroupInput"]
+    for targets, (x, y) in input_regions:
+        sockets = [(socket, name) for socket, name in inputs if socket.node in [stage[key] for key in targets if key in stage]]
+        if not sockets:
+            continue
+        control = group.nodes.new("NodeGroupInput")
+        control.location = (x + offset[0], y + offset[1])
+        for socket, name in sockets:
+            group.links.new(control.outputs[name], socket)
+    return stage
+
+
+def finish_layout(group):
+    """Expose linked fields and retain readable constants."""
+    for node in group.nodes:
+        node.select = False
+        node.width = 200 if "Attribute" in node.bl_idname else 160
+        if node.bl_idname in {"ShaderNodeMath", "ShaderNodeVectorMath"}:
+            node.hide = all(socket.is_linked for socket in node.inputs if socket.enabled and not socket.hide)
+        for socket in node.outputs:
+            socket.hide = not socket.is_linked
+    for node in list(group.nodes):
+        if node.bl_idname == "NodeGroupInput" and not any(socket.is_linked for socket in node.outputs):
+            group.nodes.remove(node)