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>

examples/HDRBloom/blur.comp.wgsl

@@ -0,0 +1,33 @@
+// HDRBloom blur pass (PlainComputeShader). Box-blurs the thresholded bloom
+// mip into a second rgba16float target — demonstrating an N-target float
+// chain: this pass SAMPLES the texture the threshold pass wrote (storage
+// write → sampled read across a submit barrier) and WRITES another
+// user-owned float storage texture. Two such targets prove the mip-pyramid
+// shape a real bloom needs.
+//
+// Layout mirrors threshold.comp.wgsl (src = bloom mip A, dst = bloom mip B).
+
+struct Dim { w: u32, h: u32, _0: u32, _1: u32 };
+@group(0) @binding(0) var<uniform> dim : Dim;
+@group(1) @binding(0) var src : texture_2d<f32>;
+@group(1) @binding(1) var dst : texture_storage_2d<rgba16float, write>;
+
+const R: i32 = 6;   // box radius → 13×13 kernel; a wide, cheap glow
+
+@compute @workgroup_size(8, 8, 1)
+fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
+    if (gid.x >= dim.w || gid.y >= dim.h) { return; }
+    let maxX = i32(dim.w) - 1;
+    let maxY = i32(dim.h) - 1;
+    var sum = vec3<f32>(0.0);
+    var count = 0.0;
+    for (var dy = -R; dy <= R; dy = dy + 1) {
+        for (var dx = -R; dx <= R; dx = dx + 1) {
+            let x = clamp(i32(gid.x) + dx, 0, maxX);
+            let y = clamp(i32(gid.y) + dy, 0, maxY);
+            sum = sum + textureLoad(src, vec2<i32>(x, y), 0).rgb;
+            count = count + 1.0;
+        }
+    }
+    textureStore(dst, vec2<i32>(i32(gid.x), i32(gid.y)), vec4<f32>(sum / count, 1.0));
+}