Replace the blit cascade in record_blur with shader-based dual-Kawase,

matching the OpenGL backend's algorithm. The blit cascade was a much weaker
   filter that degenerated badly on thin surfaces.

   The new path adds blur.vert / blur_down.frag (5-tap) / blur_up.frag (8-tap),
   a single-binding blur descriptor set layout, and per-format down/up pipelines.
   BLUR_SCRATCH_USAGE gains COLOR_ATTACHMENT so the scratch images can be both
   sampled and rendered into. Cache hit fast path and masked composite are
   unchanged.

build/vulkan/hash.rs

@@ -25,6 +25,9 @@ pub const TREES: &[Tree] = &[Tree {
         "rounded_tex.vert",
         "blur_composite.vert",
         "blur_composite.frag",
+        "blur.vert",
+        "blur_down.frag",
+        "blur_up.frag",
         "legacy/fill.frag",
         "legacy/fill.vert",
         "legacy/tex.vert",