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>

CMakeLists.txt

@@ -13,6 +13,7 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/modules")
 find_package(OpenGL REQUIRED)
 find_package(OpenUSD REQUIRED)
 find_package(Imgui REQUIRED)
+find_package(ImguiNodeEditor REQUIRED)
 find_package(Glad REQUIRED)
 find_package(FFmpeg REQUIRED)
 
@@ -195,6 +196,7 @@ target_include_directories(UsdLayerManager PRIVATE
 target_link_libraries(UsdLayerManager PRIVATE
     OpenUSD::OpenUSD
     Imgui::Imgui
+    ImguiNodeEditor::ImguiNodeEditor
     Glad::Glad
     FFmpeg::FFmpeg
     OpenColorIO::OpenColorIO
@@ -577,6 +579,33 @@ if(NOT EXISTS "${_ocio_config_dir}/config.ocio")
     endif()
 endif()
 
+# ---------------------------------------------------------------------------
+# HDRI: shader-ball lighting presets (Poly Haven, CC0) at configure time
+# ---------------------------------------------------------------------------
+set(_hdri_dir "${CMAKE_SOURCE_DIR}/resources/hdri")
+set(_hdri_files
+    kloofendal_48d_partly_cloudy_puresky_1k.exr
+    lebombo_1k.exr
+    artist_workshop_1k.exr
+    venice_sunset_1k.exr
+)
+file(MAKE_DIRECTORY "${_hdri_dir}")
+foreach(_hdri ${_hdri_files})
+    if(NOT EXISTS "${_hdri_dir}/${_hdri}")
+        message(STATUS "Downloading HDRI ${_hdri} ...")
+        file(DOWNLOAD
+            "https://dl.polyhaven.org/file/ph-assets/HDRIs/exr/1k/${_hdri}"
+            "${_hdri_dir}/${_hdri}"
+            STATUS _hdri_dl_status
+        )
+        list(GET _hdri_dl_status 0 _hdri_dl_code)
+        if(NOT _hdri_dl_code EQUAL 0)
+            file(REMOVE "${_hdri_dir}/${_hdri}")
+            message(WARNING "HDRI download failed for ${_hdri} (${_hdri_dl_status}) — the shader-ball lighting preset will fall back to a distant light.")
+        endif()
+    endif()
+endforeach()
+
 # Copy resources to build directory
 file(COPY ${CMAKE_SOURCE_DIR}/resources
     DESTINATION ${CMAKE_BINARY_DIR}
@@ -592,7 +621,10 @@ add_custom_command(TARGET UsdLayerManager POST_BUILD
     COMMAND ${CMAKE_COMMAND} -E copy_directory
         "${CMAKE_SOURCE_DIR}/resources/icons"
         "$<TARGET_FILE_DIR:UsdLayerManager>/resources/icons"
-    COMMENT "Copying fonts and SVG icons to build output"
+    COMMAND ${CMAKE_COMMAND} -E copy_directory
+        "${CMAKE_SOURCE_DIR}/resources/hdri"
+        "$<TARGET_FILE_DIR:UsdLayerManager>/resources/hdri"
+    COMMENT "Copying fonts, SVG icons and HDRI presets to build output"
 )
 
 # Copy ACES OCIO config to build output — only on first build (skipped if already present).