feat(vulkan-rt): BLAS build options — fast-build/fast-trace + in-place refit (#36)

Mesh::Build / BuildProcedural now take an RTBuildOptions { preference,
allowUpdate }. `preference` maps to PREFER_FAST_TRACE (default) or
PREFER_FAST_BUILD; `allowUpdate` sets ALLOW_UPDATE so the BLAS can be
refit later.

Adds Mesh::Refit / RefitProcedural: when the original build opted into
allowUpdate and the topology is unchanged, they record an in-place
UPDATE-mode build (src == dst) — much cheaper than a rebuild, and the
AS handle + blasAddr are preserved so TLAS instances stay valid. They
fall back to a full rebuild otherwise. The shared build tail also now
destroys a stale AS handle on re-Build (previously leaked) and guards
the in-place update with an AS read→write barrier.

The portable RTBuildPreference/RTBuildOptions types and Refit methods
also exist on the WebGPU backend for API symmetry; the software BVH has
no hardware AS, so the preference is a no-op and a "refit" rebuilds the
BVH (re-registering the handle).

Also adds Device::validationErrorCount, bumped by the debug-messenger
callback on ERROR-severity messages, so tests can assert a Vulkan
operation produced no validation errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

implementations/Crafter.Graphics-Device.cpp

@@ -167,6 +167,7 @@ VkBool32 onError(VkDebugUtilsMessageSeverityFlagBitsEXT severity, VkDebugUtilsMe
         break;
     case VK_DEBUG_UTILS_MESSAGE_SEVERITY_ERROR_BIT_EXT :
         printf("(error): ");
+        Device::validationErrorCount++;
         break;
     }