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/templates/ugc.jinja2

@@ -35,8 +35,8 @@
 	<details class="card mb-3" id="optionsCard">
 		<summary class="card-header py-2 small fw-semibold">Processing options compared</summary>
 		<div class="card-body py-2 small">
-			<p class="text-body-secondary mb-2">The ways of making models to compare: the ray tracer of the occlusion (Embree on the CPU, HIPRT on the
-				GPU) and denoising of icons. The settings pick the defaults; <b>Make again</b> can use others for a make (the options next
+			<p class="text-body-secondary mb-2">The ways of making models to compare: the ray tracer of the occlusion (Embree on the CPU, HIPRT on AMD or
+				NVIDIA GPUs, Embree on Intel GPUs) and denoising of icons. The settings pick the defaults; <b>Make again</b> can use others for a make (the options next
 				to it, or <code>/reprocessproperty embree oidn</code> in game).
 				Every make is counted under what made it: the averages per make below, each model's in the List view's Options column.</p>
 			<div class="table-responsive"><table class="table table-sm align-middle mb-0" id="optionsTable">