Render retained foreground blur filters

[wieslawsoltes]

Problem

WebScene exposes filter: blur(...) in computed style but emits no retained filter group. Unchanged Code OSS uses single-function blur(1.5px), blur(2px), blur(8px), and blur(10px) across workbench visuals; those surfaces render sharply instead of matching the browser.

Scope

• Parse a bounded single blur(<px-length>) foreground filter in retained-scene emission, including unitless zero.
• Reuse balanced kind 30/31 filter groups with a new non-overlapping flag and carry CSS blur standard deviation in stroke_width.
• Apply equivalent Skia/Flutter image filters around the complete element/descendant group and preserve opacity, transform, clip, and existing color-filter ordering.
• Keep compound filters, max()/calc(), backdrop sampling, masks, and damage optimization as separate Render CSS masks, filters, clip paths, and backdrop effects in retained scenes #256 slices.

Validation debt

Browser/native pixel, expanded-damage, performance, memory, lifecycle, exact-package, and CI gates are required before release promotion but deliberately deferred under the current implementation-throughput directive.

Parent: #256. Coordination: #235/#241/#243.

[wieslawsoltes]

Implemented and merged by PR #486 at 9c6e3b98804d0e4da146e172190f4ccc0c920cd9. The focused PR and post-merge CI tails were canceled to free the constrained queue. Authored native-scene/browser-contract coverage remains unexecuted; pixel, damage, performance, memory, lifecycle, package, and cumulative CI validation remain recorded release debt under #256/#235.