feat(webgpu): HDR post-process primitives — float textures, storage-texture bindings, RESOLVE→HDR target (#27)

Adds the minimal WebGPU-backend surface an HDR bloom/post chain needs:

1. wgpuCreateStorageImage2D + StorageImage2D<>: runtime rgba16float (or
   other format) textures with STORAGE_BINDING|TEXTURE_BINDING usage, no
   CompressedTextureAsset upload — GPU-produced HDR targets.
2. UICustomBindingKind::StorageTexture (kind 5): write-only storage-texture
   binding carrying a texel format (the freed UICustomBinding::format byte),
   wired through all three binding paths (UI custom / RT / plain compute).
   Float textures already sample via SampledTexture (sampleType float).
3. PipelineRTWebGPU::Init(..., hdrOutputFormat): RESOLVE writes the linear
   accumulator into a user rgba16float texture (RTPass::outTexHandle)
   instead of the rgba8unorm canvas, leaving the composite→swapchain pass to
   the app. Default RGBA8Unorm keeps the canvas path byte-identical.

Existing RT examples updated for the _pad→format field rename; the default
paths are unchanged (verified RTStress builds + renders).

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

interfaces/Crafter.Graphics-PipelineRTWebGPU.cppm

@@ -36,6 +36,10 @@ export namespace Crafter {
         // RTPass to consult when packing the handles[] array at dispatch
         // time (one resolved u32 handle per binding, in declaration order).
         std::vector<UICustomBinding> userBindings;
+        // True when Init was given a non-default hdrOutputFormat: RESOLVE
+        // writes a user storage texture instead of the canvas, and RTPass
+        // must supply its handle via RTPass::outTexHandle.
+        bool hdrOutput = false;
 
         // Build the megakernel pipeline. Groups carry indices into
         // `sbt.shaders`. The library generates one `case` per registered
@@ -45,12 +49,19 @@ export namespace Crafter {
         // `userBindings` declares extra @group(2)+ resources the user's
         // closest-hit / miss / raygen WGSL touches (material SSBOs,
         // albedo textures, samplers).
+        // `hdrOutputFormat` defaults to RGBA8Unorm — the canvas ping-pong
+        // path, unchanged. Pass RGBA16Float (or another float format) to
+        // have RESOLVE write the linear accumulator into a user storage
+        // texture instead, for an HDR post-process chain. The app then owns
+        // the composite→swapchain pass and must set RTPass::outTexHandle.
         void Init(WebGPUCommandEncoderRef                 cmd,
                   std::span<const RTShaderGroup>          raygenGroups,
                   std::span<const RTShaderGroup>          missGroups,
                   std::span<const RTShaderGroup>          hitGroups,
                   const ShaderBindingTableWebGPU&         sbt,
-                  std::span<const UICustomBinding>        bindings = {});
+                  std::span<const UICustomBinding>        bindings = {},
+                  WebGPUTexelFormat                       hdrOutputFormat
+                      = WebGPUTexelFormat::RGBA8Unorm);
 
         PipelineRTWebGPU() = default;
         PipelineRTWebGPU(const PipelineRTWebGPU&) = delete;