Render retained foreground blur filters

[wieslawsoltes]

Problem

Unchanged Code OSS uses single-function foreground blur filters, but WebScene published no retained blur group. Those workbench surfaces rendered sharply even though computed style exposed blur(...).

Change

• parse a bounded single blur(<px-length>), including unitless zero
• publish balanced retained filter groups with the CSS blur standard deviation
• apply equivalent Skia and Flutter image filters around the full element subtree
• preserve transform, clip, opacity, and existing color-filter nesting
• add native-scene and browser-contract regression coverage

Compound filters, functional lengths, backdrop sampling, masks, and damage expansion remain tracked by #256.

Validation

Tests and builds were deliberately not executed under the current implementation-throughput directive. Native pixels, browser comparison, damage, performance, memory, lifecycle, package, and CI validation remain required before release promotion and are recorded in #485/#256/#235.

Closes #485