feat(ugc): denoise=oidn traces icons' occlusion per pixel and denoises it (optional build)

Intel Open Image Denoise 2 (Apache-2.0) behind the CMake option DLU_OIDN (off):
an installed OIDN is used when found, else Intel's release package (pinned by
hash) is downloaded and its libraries copied next to the servers.

A denoiser only removes noise that differs from pixel to pixel; the icons'
occlusion comes from the bake, per vertex, which it leaves as it is (checked:
white noise 0.17 -> 0.006 relative spread, per-vertex blocks unchanged). So
with denoise=oidn a model's icon is drawn from model.noao.nif (its colors
before the bake) with its occlusion traced per pixel of the supersampled image
(denoise_samples rays, default 4, with the bake's distance and strength and the
ray backend), box filtered and denoised at the icon's size, guided by the colors
and normals. The model keeps its baked occlusion. Icons drawn again from stored
files use the stored model.noao.nif the same way.

OIDN works on a thread of its own; its time is charged to the job's thread
(UgcThrottle::Charge), so the CPU budget and the recorded CPU time include it.

Check: configure with -DDLU_OIDN=ON; UgcServer --make-model x.lxfml out oidn
and compare its icon.png with one made with off; an OFF build leaves icons as
they were.

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

dDashboardServer/routes/SettingsCatalog.cpp

@@ -504,7 +504,8 @@ namespace {
 		c.Add(Labels(Choice(UGC, "hsr_method", "How hidden faces are found", "toolbox: LU Toolbox's paths (above). fast: the model rendered from 42 directions around it, removing what shows in none; much faster, but it also removes faces seen only by bounced light (insides seen through openings, recesses). Staff can pick another for one make when making models again.", "toolbox", { "toolbox", "fast" }), { "LU Toolbox (paths)", "Fast (renders from around)" }));
 		c.Add(Unit(Int(UGC, "hsr_fast_resolution", "Fast method's render size", "The size of each of the fast method's 42 renders; bigger keeps smaller visible faces.", "1024", 64, 4096), "pixels"));
 		c.Add(Labels(Choice(UGC, "ray_backend", "Ray tracer", "What traces the rays of the hidden faces' paths and of the occlusion: builtin (the UGC server's own), embree (Intel Embree on the CPU) or hiprt (the GPU, when the server was built with it and has one; else embree). They give the same results but for rounding. Staff can pick another for one make when making models again.", "builtin", { "builtin", "embree", "hiprt" }), { "Built in", "Embree (CPU)", "HIPRT (GPU)" }));
-		c.Add(Labels(Choice(UGC, "denoise", "Denoise icons", "oidn: icons are cleaned up with Intel Open Image Denoise (when the server was built with it; else off).", "off", { "off", "oidn" }), { "Off", "Open Image Denoise" }));
+		c.Add(Labels(Choice(UGC, "denoise", "Denoise icons", "oidn (when the server was built with Intel Open Image Denoise; else off): a model's icon is drawn from its colors before the occlusion bake, with its occlusion traced per pixel with a few rays and the noise removed by the denoiser, instead of the baked occlusion. The model itself keeps its baked occlusion (a denoiser only works on images).", "off", { "off", "oidn" }), { "Off", "Open Image Denoise" }));
+		c.Add(Int(UGC, "denoise_samples", "Denoised occlusion rays per pixel", "Occlusion rays traced from each pixel of a denoised icon (before it is scaled down, so 16 times as many per icon pixel); fewer is faster and noisier.", "4", 1, 256));
 		c.Add(Bool(UGC, "bake_ao", "Darken hidden corners", "Ambient occlusion baked into the vertex colors.", true));
 		c.Add(Int(UGC, "ao_samples", "Occlusion rays per vertex", "", "64", 1, 1024));
 		c.Add(Float(UGC, "ao_distance", "Occlusion distance", "", "5", 0, 1000));