Add preview shape picker to shader ball; drag controls for node properties

Preview geometry dropdown: Sphere / Cube / Cylinder (native prims),
Teapot and Hair (bundled resources/preview/*.usdc - teapot tessellated
from the Newell patch data with the Blinn 1.3x height correction, hair
as 220 procedural B-spline strands), and Cloud Volume (UsdVolVolume
over the OpenVDB bunny_cloud sample, downloaded at CMake configure and
gitignored like the HDRIs). Shapes live under one scope with the
material bound on it; switching activates one shape and reframes the
camera. Missing assets drop out of the dropdown. usdVol added to the
USD link set. The camera starts in a Maya-style 3/4 view.

Node property values switch from Input* fields back to draggable
controls (Ctrl+click to type): live-apply while dragging, one undo
command on release.

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

CMakeLists.txt

@@ -606,6 +606,33 @@ foreach(_hdri ${_hdri_files})
     endif()
 endforeach()
 
+# ---------------------------------------------------------------------------
+# VDB: cloud volume for the shader-ball preview (OpenVDB sample model) at
+# configure time if not present
+# ---------------------------------------------------------------------------
+set(_vdb_dir "${CMAKE_SOURCE_DIR}/resources/vdb")
+file(MAKE_DIRECTORY "${_vdb_dir}")
+if(NOT EXISTS "${_vdb_dir}/bunny_cloud.vdb")
+    message(STATUS "Downloading OpenVDB sample bunny_cloud.vdb ...")
+    set(_vdb_zip "${CMAKE_BINARY_DIR}/bunny_cloud.zip")
+    file(DOWNLOAD
+        "https://artifacts.aswf.io/io/aswf/openvdb/models/bunny_cloud.vdb/1.0.0/bunny_cloud.vdb-1.0.0.zip"
+        "${_vdb_zip}"
+        STATUS _vdb_dl_status
+    )
+    list(GET _vdb_dl_status 0 _vdb_dl_code)
+    if(_vdb_dl_code EQUAL 0)
+        execute_process(
+            COMMAND ${CMAKE_COMMAND} -E tar xf "${_vdb_zip}"
+            WORKING_DIRECTORY "${_vdb_dir}"
+        )
+        file(REMOVE "${_vdb_zip}")
+        message(STATUS "bunny_cloud.vdb installed to ${_vdb_dir}")
+    else()
+        message(WARNING "OpenVDB sample download failed (${_vdb_dl_status}) — the Cloud Volume preview shape will be unavailable.")
+    endif()
+endif()
+
 # Copy resources to build directory
 file(COPY ${CMAKE_SOURCE_DIR}/resources
     DESTINATION ${CMAKE_BINARY_DIR}
@@ -624,7 +651,13 @@ add_custom_command(TARGET UsdLayerManager POST_BUILD
     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"
+    COMMAND ${CMAKE_COMMAND} -E copy_directory
+        "${CMAKE_SOURCE_DIR}/resources/preview"
+        "$<TARGET_FILE_DIR:UsdLayerManager>/resources/preview"
+    COMMAND ${CMAKE_COMMAND} -E copy_directory
+        "${CMAKE_SOURCE_DIR}/resources/vdb"
+        "$<TARGET_FILE_DIR:UsdLayerManager>/resources/vdb"
+    COMMENT "Copying fonts, SVG icons, HDRI presets and preview shapes to build output"
 )
 
 # Copy ACES OCIO config to build output — only on first build (skipped if already present).