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/ui/Application.h

@@ -8,9 +8,11 @@
 #include "PropertyPanel.h"
 #include "TimelinePanel.h"
 #include "CurveEditorPanel.h"
+#include "MaterialEditorPanel.h"
 #include "../core/UsdStageManager.h"
 #include "../core/LayerManager.h"
 #include "../core/PropertyManager.h"
+#include "../core/MaterialManager.h"
 #include "../core/CommandHistory.h"
 #include "../utils/MovieEncoder.h"
 #include <memory>
@@ -70,6 +72,7 @@ private:
     std::unique_ptr<UsdStageManager> m_stageManager;
     std::unique_ptr<LayerManager> m_layerManager;
     std::unique_ptr<PropertyManager> m_propertyManager;
+    std::unique_ptr<MaterialManager> m_materialManager;
     CommandHistory m_commandHistory;
     std::unique_ptr<StageEditorPanel> m_stageEditorPanel;
     std::unique_ptr<SceneHierarchyPanel> m_sceneHierarchyPanel;
@@ -77,6 +80,7 @@ private:
     std::unique_ptr<PropertyPanel> m_propertyPanel;
     std::unique_ptr<TimelinePanel> m_timelinePanel;
     std::unique_ptr<CurveEditorPanel> m_curveEditorPanel;
+    std::unique_ptr<MaterialEditorPanel> m_materialEditorPanel;
     bool m_showDemoWindow;
     bool m_showStageInfo;
     bool m_showStageEditor;
@@ -85,6 +89,7 @@ private:
     bool m_showPropertyPanel;
     bool m_showTimeline;
     bool m_showCurveEditor;
+    bool m_showMaterialEditor;
     bool m_running;
 
     MovieEncoder   m_movieEncoder;