Add Hypershade-style Material Editor with node graph and shader-ball preview

Browser column lists all scene materials plus a searchable create-node
list; Show Graph (or double-click) loads a material into the node-graph
work area. The graph shows the entire network including MaterialX
(.mtlx) node graphs, resolving connections through NodeGraph boundaries
via UsdShadeUtils::GetValueProducingAttributes, with layered auto-layout
for nodes lacking authored uiPosition. Canvas navigates viewport-style
(Alt+MMB pan / Alt+RMB zoom) and TAB opens a Nuke-style search popup.
Selecting a node shows a typed property editor (live-apply, one undo
command per edit) and previews that node's output on the shader ball.

The preview renders through its own Hydra engine into a scratch stage
that composes the material via a reference to the source root layer (so
referenced .mtlx materials work), re-renders until progressive delegates
(Arnold/Cycles/Embree) converge, and lights with HDR dome presets
(External/Room/Interior/Sunset; CC0 Poly Haven EXRs fetched at CMake
configure). texture:format is authored latlong explicitly - left
automatic, hdArnold falls back to Arnold's angular fisheye default -
and hdCycles gets a -90 X pole rotation to match Storm's +Y-pole
sampling.

Mutations go through new ICommand subclasses (create shader node,
connect/disconnect attrs). imgui-node-editor is vendored (gitignored)
with a local one-line patch: c_ScrollButtonIndex 1->2 for MMB pan.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/core/commands/DisconnectShaderAttrCommand.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include "../CommandHistory.h"
+#include <pxr/usd/usd/stage.h>
+#include <pxr/usd/sdf/path.h>
+#include <string>
+
+namespace UsdLayerManager {
+
+/// Disconnects a shader input from its upstream source, capturing that
+/// source at construction time so Undo can restore it.
+class DisconnectShaderAttrCommand : public ICommand {
+public:
+    DisconnectShaderAttrCommand(pxr::UsdStageRefPtr stage,
+                                const pxr::SdfPath&  destNode,
+                                const std::string&   destInput);
+
+    void Execute() override;
+    void Undo()    override;
+    std::string GetDescription() const override { return m_description; }
+
+private:
+    pxr::UsdStageRefPtr m_stage;
+    pxr::SdfPath        m_destNode;
+    std::string         m_destInput;
+    std::string         m_description;
+
+    bool         m_hadConnection = false;
+    pxr::SdfPath m_sourceNode;
+    std::string  m_sourceOutput;
+};
+
+} // namespace UsdLayerManager