test(webgpu): HDRBloom example exercising the HDR post-process primitives (#27)

RT→rgba16float→threshold→blur→composite, end-to-end proof of the three
primitives. threshold/blur run from onBeforeUpdate (one submit each) so the
storage-write→sampled-read dependency gets WebGPU's per-submit barrier
(there is none between dispatches within a compute pass); the composite is a
UI custom shader so it owns the canvas ping-pong. Documents the Vulkan
symmetry (gap 4): the native present path records passes generically and
barriers between them, so the same chain is wireable today.

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

WAVEFRONT-DESIGN.md

@@ -42,6 +42,16 @@ Accum buffer is linear. Optional user `WebGPURTStage::Resolve` entry
 `resolve_main(coord:vec2<u32>, hdr:vec4<f32>)->vec4<f32>`. None → passthrough.
 VulkanTriangle: no resolve (exact match). Sponza: resolve does Reinhard+gamma.
 
+### HDR output target (issue #27)
+`PipelineRTWebGPU::Init(..., hdrOutputFormat)` (default `RGBA8Unorm` = the
+canvas ping-pong path, unchanged) can instead point RESOLVE at a user
+`rgba16float` storage texture (`RTPass::outTexHandle`). The JS side swaps
+binding(6)'s WGSL declaration + bind-group-layout format and skips the
+ping-pong flip, since the canvas is untouched. With no resolve shader the
+default passthrough writes raw linear radiance — the HDR input an app's own
+bloom/composite chain (threshold → blur → tonemap → swapchain) reads. See
+`examples/HDRBloom`.
+
 ## Indirect dispatch (Phase 2 de-risk)
 Prove `dispatchWorkgroupsIndirect` + cross-pass atomic visibility with a toy
 "emit N → dispatch N" before wiring real kernels. WebGPU inserts an implicit