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/UsdSceneRenderer.h

@@ -185,12 +185,23 @@ public:
     /// unknown or the switch fails.  The renderer is re-initialised if needed.
     bool SetRendererPlugin(const pxr::TfToken& pluginId);
 
+    /// False while a progressive delegate (Arnold/Cycles/Embree…) is still
+    /// accumulating samples for the last-submitted frame; Storm is always
+    /// converged after one pass. Callers that cache the output texture must
+    /// keep calling Render() until this is true.
+    bool IsConverged() const { return !m_renderer || m_renderer->IsConverged(); }
+
     // -----------------------------------------------------------------------
     // View settings
     // -----------------------------------------------------------------------
     bool              ShowGrid()                const { return m_showGrid; }
     void              SetShowGrid(bool show)          { m_showGrid = show; }
 
+    /// Hydra refinement complexity (1.0 = base mesh, up to 2.0). The shader
+    /// ball uses a raised value so the sphere tessellates smoothly.
+    float             Complexity()              const { return m_complexity; }
+    void              SetComplexity(float c)          { m_complexity = c; }
+
     bool              ShowCameraGuide()         const { return m_showCameraGuide; }
     void              SetShowCameraGuide(bool show)   { m_showCameraGuide = show; }
 
@@ -360,6 +371,7 @@ private:
     std::vector<pxr::GfVec4d> m_clipPlanes;
 
     bool            m_showGrid;
+    float           m_complexity = 1.0f;
     bool            m_showCameraGuide;
     bool            m_showGuides;
     bool            m_showProxy;