feat(ugc): ray_backend=embree-gpu traces on Intel GPUs with Embree's SYCL (optional build)

The third ray backend, so every machine has a library for it: Embree on x86
CPUs (embree), HIPRT on AMD and NVIDIA GPUs (hiprt), Embree through SYCL on
Intel Arc and Xe GPUs (embree-gpu).

- CMake option DLU_EMBREE_SYCL (off). dUgcServer/EmbreeSycl is a project of
  its own built by a SYCL compiler (DLU_SYCL_CXX; icpx through ONEAPI_ROOT or
  the path, or the open source DPC++'s clang++ through DPCPP_ROOT) as an
  external project: Embree 4.4 with EMBREE_SYCL_SUPPORT, linked statically and
  bound inside (-Bsymbolic, only its C functions exported, so it never meets
  the servers' own Embree), and the GPU kernels (nearest hit skipping a ray's
  triangle, any hit), into libdlu_embree_sycl next to the servers
- UgcRaysEmbreeGpu loads it the first time embree-gpu is asked for; one GPU for
  the process (embree_gpu_device picks it), the workers take turns, the
  occlusion rays in batches as for hiprt
- without the build, the library or a supported Intel GPU it falls back to
  embree and says why (the UGC server's log at start, --make-model on stderr)
- the option names, the settings page, the dashboard's picker, /reprocessproperty

Verified here: the default build and ctest; the SYCL build with the open source
DPC++ 7.1.0 (compiles, links against oneAPI's libsycl.so.9, exports only its C
functions); on this machine (no Intel GPU) it loads, finds no GPU and falls
back to embree. Not verified: tracing on an Intel GPU (none here).

Check: on a machine with an Intel Arc or Xe GPU and oneAPI, configure with
-DDLU_EMBREE_SYCL=ON and run UgcServer --make-model x.lxfml out embree-gpu;
the UGC tests then compare it with Embree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dDashboardServer/static/js/ugc.js

@@ -727,7 +727,7 @@
 
 	// ---- processing options: Make again's choices and the comparison of what made the models ----
 
-	var OPTION_LABELS = { embree: 'Embree (CPU)', hiprt: 'HIPRT (GPU)', off: 'Off', oidn: 'Open Image Denoise' };
+	var OPTION_LABELS = { embree: 'Embree (CPU)', hiprt: 'HIPRT (GPU)', 'embree-gpu': 'Embree (Intel GPU)', off: 'Off', oidn: 'Open Image Denoise' };
 	function loadOptions() {
 		return api.get('/api/ugc/options').then(function (d) {
 			if (!d.success) throw new Error(d.error || 'Failed');