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-WebGPUComputeShader.cppm

@@ -31,6 +31,16 @@ import std;
 import :WebGPU;
 
 export namespace Crafter {
+    // Texel format for runtime-allocated textures and write-only storage-
+    // texture bindings. Values are the wire contract with dom-webgpu.js's
+    // texelFormatStr(); keep them in sync. rgba8unorm (0) is the default so
+    // an unset `format` byte preserves prior behaviour.
+    enum class WebGPUTexelFormat : std::uint8_t {
+        RGBA8Unorm  = 0,
+        RGBA16Float = 1,   // HDR post-process targets — filterable per spec
+        R32Float    = 2,
+    };
+
     enum class UICustomBindingKind : std::uint8_t {
         Buffer              = 0,   // read-only-storage SSBO, handle is a slot into heap.bufferTable
         SampledTexture      = 1,   // sampled texture_2d<f32>, handle is a slot into heap.imageTable
@@ -41,13 +51,20 @@ export namespace Crafter {
         // integrate node momentum, write brace stress, or output TLAS
         // instance transforms.
         BufferReadWrite     = 4,
+        // write-only storage texture (texture_storage_2d<FMT, write> in
+        // WGSL). The `format` field carries the WebGPUTexelFormat. Lets a
+        // compute pass write a user-owned texture (e.g. an rgba16float bloom
+        // mip) at an arbitrary group/binding — the missing primitive for an
+        // HDR mip pyramid. Bind the same texture as SampledTexture to read
+        // it back in a later pass.
+        StorageTexture      = 5,
     };
 
     struct UICustomBinding {
         std::uint8_t  group;        // @group(N), must be >= 2 (0 and 1 are reserved)
         std::uint8_t  binding;      // @binding(N)
         UICustomBindingKind kind;
-        std::uint8_t  _pad;
+        std::uint8_t  format;       // WebGPUTexelFormat for StorageTexture; 0 otherwise
         std::uint32_t pushOffset;   // offset in push data where the slot uint32 lives
     };
     static_assert(sizeof(UICustomBinding) == 8);