fix(rt): split Sponza's combined image sampler into a heap texture and sampler

examples/Sponza no longer built natively (#164). Its closest-hit declared a
descriptor_heap sampler2D, which glslang has rejected since c8253623
("Reject descriptor_heap combined image samplers"), and main.cpp wrote it
through VkResourceDescriptorDataEXT::pCombinedImageSampler, which that union
no longer has: the descriptor heap has no combined image samplers. Both went
unnoticed because nothing builds the examples.

Sponza
- closesthit.glsl declares texture2D and sampler heap arrays and joins them
  where it samples, as ui-shared.glsl and HeapDynamicIndexHit do. The
  sampler slot is a second spec constant beside the image slot, so the
  shader assumes nothing about where the host put it, and a constant index
  keeps the example running on NVIDIA 610.43.02 (see its README). The
  comment no longer says the image slot is offset by bufferStartElement;
  only buffer slots are.
- main.cpp gives the heap a sampler slot, writes the albedo as a
  SAMPLED_IMAGE from a view create-info, and writes the sampler into the
  sampler heap from a VkSamplerCreateInfo with vkWriteSamplerDescriptorsEXT,
  as UIRenderer::RegisterSampler does. SamplerVulkan's VkSampler had no use
  left. The sampler keeps its linear filtering and repeat addressing and
  drops anisotropy, which a hit shader cannot use: it samples at LOD 0,
  without derivatives.

Docs
- README.md and examples/Sponza/README.md quote the new declarations, and
  the Sponza README says why the texture and the sampler are two
  descriptors. Its texture2DArray materials pattern declared a
  descriptor_heap sampler2DArray, which glslang rejects as well; it now
  joins a texture2DArray and a sampler.
- examples/SponzaBench/README.md drops its caveat that Sponza does not
  compile.

Tests
- ExampleShaders compiles every GLSL shader under examples/ with glslang as
  crafter-build does for the example (Vulkan rules, SPIR-V 1.4, shaders/ on
  the include path, and the descriptor-set build where the example
  registers one), so a glslang that starts rejecting one fails a test
  rather than an example build nobody runs. It fails on Sponza's old
  closest-hit and needs glslang only.

Closes #164.

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

examples/SponzaBench/README.md

@@ -119,13 +119,6 @@ scene for the dynamic-upload PRs) would be needed.
 
 ## Caveats / notes
 
-* The stock `examples/Sponza` **native** path does not currently compile
-  against the (unpinned) Vulkan-Headers `main`: the
-  `VkResourceDescriptorDataEXT` union no longer has the
-  `pCombinedImageSampler` member the example uses. SponzaBench sidesteps
-  this by not binding a combined image+sampler. (Pre-existing, unrelated to
-  #155 — worth a follow-up to port Sponza to the split sampled-image +
-  sampler-heap model the engine's own UI renderer already uses.)
 * Numbers are CPU/GPU specific. Re-run `run-bench.sh` locally for your
   hardware; the *deltas* between revisions are the point, not the absolute
   figures.