Interpolate bounded foreground filters on the host clock #537

Parent: #242. CSS epic: #235. Release epic: #227.

Proven gap

Unchanged Code OSS uses CSS Transitions between grayscale(1) and none on Chat/agent/session icons and between none and brightness(1.2) on Chat debug hover. WebScene paints bounded foreground filter lists but does not interpolate them or keep host frame demand active.

Focused implementation

• Reuse one typed retained foreground-filter list for parsing, transition state, computed style, and paint.
• Preserve authored order and the existing 16-function/value bounds for blur, brightness, contrast, grayscale, and saturate.
• Interpolate only compatible same-order lists, padding prefixes and none with function-specific identities.
• Advance from host timestamps, emit run/start/cancel/end, support reversal, and release frame demand after completion.
• Keep transitions paint-only and preserve stacking context until a final none clears the identity list.
• Account retained filter state in allocation/certification metrics.
• Add native temporal/CSSOM/scene/layout/event/reversal/idle coverage, a 128-element browser contract, and a midpoint pixel reftest.

Acceptance and deferred gates

• Implementation rebased on WebScene main e33fdbc6c11fd292070b5c8442568da39d39c57d.
• Focused regression sources authored.
• git diff --check passed.
• Execute Chromium/native temporal, pixel, performance, allocation, lifecycle, and idle gates.
• Run exact Code OSS and package acceptance.

Filter keyframes, WAAPI, additional functions, and mismatched/reordered-list interpolation remain outside this slice.