feat: Add new Geometry Nodes groups for image relief and shadeless shader

- Implemented the "O Image Relief Plane" Geometry Nodes group to create a fixed-base image relief using depth images and various parameters.
- Created the "O Shadeless" shader node group to display color without scene lighting while preserving shadows and reflections.
- Introduced a preview feature for node groups, allowing for interactive visualization of node arrangements in an HTML format.
- Added a template HTML file for rendering node previews with customizable views and controls.

tests/node_asset_test_support.py

@@ -0,0 +1,22 @@
+"""Shared assertions and asset selection for node tests."""
+from tools.nodes.groups.image_cutout import build_image_cutout_group
+from tools.nodes.groups.image_depth_cutout import build_image_depth_cutout_group
+from tools.nodes.groups.image_depth_balloon import build_image_depth_balloon_group
+
+CUTOUT_BUILDERS = {
+    "FLAT": build_image_cutout_group,
+    "SOLID": build_image_cutout_group,
+    "DEPTH_SOLID": build_image_depth_cutout_group,
+    "DEPTH_BALLOON": build_image_depth_balloon_group,
+}
+
+
+def validate_node_structure(node_group):
+    """Check visible sockets, default titles, and paired execution endpoints."""
+    nodes = [node for node in node_group.nodes if node.bl_idname not in {"NodeReroute", "NodeFrame"}]
+    assert all(not node.label for node in nodes), node_group.name
+    assert not any(node.bl_idname == "FunctionNodeInputString" for node in nodes), node_group.name
+    assert all(socket.is_linked or socket.hide for node in node_group.nodes for socket in node.outputs), node_group.name
+    for node in nodes:
+        if node.bl_idname in {"GeometryNodeRepeatInput", "GeometryNodeSimulationInput", "GeometryNodeForeachGeometryElementInput"}:
+            assert node.paired_output in node_group.nodes.values(), (node_group.name, node.name)