Resolve functional blur lengths in retained filters #489

Problem

Unchanged Code OSS uses filter: blur(max(4px, calc(8px * var(--vg-size)))) for voice glow rendering. WebScene's retained filter parser stops at the first nested ) and only accepts literal px, so the valid computed filter is exposed but never painted.

Scope

• Scan filter functions with balanced parentheses and retain the existing 16-function bound.
• Resolve blur lengths through WebScene's existing computed length/calc/min/max and custom-property pipeline.
• Support non-negative absolute/font/viewport/container computed lengths while rejecting percentages and unresolved/invalid values fail-closed.
• Clamp retained blur to the existing 256 CSS-pixel safety bound.
• Preserve compound ordering, saturation, opacity, transforms, clips, descendants, and backend parity.
• Add native retained-radius and browser computed-value regression coverage using the Code OSS-shaped max(..., calc(...)) case.

Validation debt

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

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