mix-blend-mode silently kills backdrop-filter in liquid-glass layer stacks

Building an Apple-style liquid glass component (a backdrop-filter layer plus an SVG feDisplacementMap filter for edge refraction), the blur and warp rendered as flat tint with no error. Cause: a sibling grain layer had mix-blend-mode: overlay. A blended child forces its parent stacking context into an isolated group, and Chrome treats that isolation as a Backdrop Root — so the sibling backdrop-filter layer samples an empty backdrop and both the blur and the SVG filter become no-ops. Same trap applies to isolation:isolate, opacity<1, mask and filter on any ancestor of the backdrop-filtered element, and to nesting one blurred glass surface inside another (the outer becomes the inner one's backdrop root). Fixes: drop the blend mode and use plain low opacity for grain, and give nested glass a blur=false mode that leans on tint plus rim instead.