Upgrade to OpenUSD 25.11 (Python 3.12) and fix Arnold/Embree regressions

Build against OpenUSD 25.11 built from source with Python 3.12, OCIO 2.2.1,
OpenVDB, Ptex and Embree, replacing the 25.05/py311 mix.

Build system:
- CMAKE_PREFIX_PATH -> third_party/OpenUSD-v25.11; HDARNOLD_ROOT ->
  hdArnold-v25.11. hdEmbree is bundled in the USD build now, so
  HDEMBREE_USD_ROOT is no longer needed.
- FindOpenUSD: drop usd_ndr (ndr was merged into sdr in 25.11).
- hdArnold plugin renames: ndrArnold -> nodeRegistryArnold, plus the new
  usdImagingArnold plugin.
- Glob the version-suffixed OpenColorIO_*.dll instead of hardcoding 2_1.
- Drop the python311.dll workaround; USD and Cycles now share Python 3.12.
- Cycles: deploy OpenColorIO_2_5.dll (needed by its bundled OpenImageIO) and
  stop the debug-DLL filter from eating IlmThread.dll -- the regex `d[.]dll$`
  also matched legitimate Release DLLs.

Runtime fixes:
- cullStyle now defaults to Nothing, matching usdview (viewSettingsDataModel
  cullBackfaces=False). BackUnlessDoubleSided drops back faces on geometry
  that isn't authored doubleSided, which hdEmbree applies to occlusion rays
  too, so interiors shaded as single-sided.
- Set HDARNOLD_osl_includepath (and PXR_MTLX_STDLIB_SEARCH_PATHS) in main.cpp
  before the plugin DLL pre-load. hdArnold compiles MaterialX via generated
  OSL that begins with #include "mx_funcs.h"; without an include path Arnold
  fails with "fatal error: 'mx_funcs.h' file not found". It must be set before
  any plugin loads because TF_DEFINE_ENV_SETTING caches the value when
  hdArnold.dll registers its settings.
- Deploy the MaterialX standard library next to the executable.

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

src/ui/Application.cpp

@@ -177,6 +177,18 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
         }
     }
 
+    // NOTE: HDARNOLD_osl_includepath / PXR_MTLX_STDLIB_SEARCH_PATHS are set in
+    // main.cpp, NOT here.  They must be in the environment before the plugin
+    // DLLs are loaded, because TF_DEFINE_ENV_SETTING caches the value when
+    // hdArnold.dll registers its settings.  Setting them at this point is too
+    // late and is silently ignored.
+    if (const char* oslInc = getenv("HDARNOLD_osl_includepath")) {
+        LOG_INFO(std::string("Arnold OSL include path: ") + oslInc);
+    } else {
+        LOG_WARNING("HDARNOLD_osl_includepath not set — Arnold MaterialX "
+                    "shaders will fail to compile ('mx_funcs.h' not found)");
+    }
+
     // Load per-viewport settings and global preferences from AppData.
     if (const char* appData = getenv("APPDATA")) {
         namespace fs = std::filesystem;