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-RTPass.cppm

@@ -97,6 +97,12 @@ export namespace Crafter {
         // bounce; etc. The library unrolls GENERATE; (PREP; TRACE; SHADE)
         // ×maxDepth; RESOLVE.
         std::uint32_t     maxDepth     = 1;
+        // Destination storage-texture handle for an HDR-output pipeline
+        // (one Init'd with a non-default hdrOutputFormat). RESOLVE writes
+        // the linear accumulator here instead of the canvas; the app runs
+        // its own composite→swapchain pass afterwards. Ignored (0) for the
+        // default canvas path.
+        std::uint32_t     outTexHandle = 0;
 
         RTPass(PipelineRTWebGPU* p) : pipeline(p) {}
 
@@ -114,7 +120,8 @@ export namespace Crafter {
                 static_cast<std::int32_t>(gy),
                 handlesPtr,
                 static_cast<std::int32_t>(handlesCount),
-                static_cast<std::int32_t>(maxDepth));
+                static_cast<std::int32_t>(maxDepth),
+                outTexHandle);
         }
     };
 }