Animate background colors on the host frame clock #533

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

Proven gap

Unchanged Code OSS authors background-color and background transitions across editor, status, toggle, toolbar, Chat, and session surfaces. WebScene advances foreground color, opacity, transform, and positional transitions, but background paint jumps directly to its target and does not keep frame demand active.

Focused implementation

• Canonicalize transition-property: background to background-color for the supported color subset.
• Store cold per-node background timing plus painted/target RGBA.
• Resolve fixed and currentColor targets and interpolate from host timestamps.
• Expose temporal RGBA through computed style and retained scene paint.
• Reuse transition run/start/cancel/end semantics and cancel on property removal.
• Include active background transitions in frame demand while keeping the path paint-only and layout-clean.
• Preserve existing detach/display-none lifecycle and idle filtering.
• Extend native and browser temporal contracts for aliasing, midpoint, reversal, final value, events, bounded samples, and post-completion idle demand.

Acceptance and deferred gates

• Implementation rebased on WebScene main a5131262914c29bc94a8755b3a6ae2d14479488f.
• Focused regression sources authored.
• git diff --check passed.
• Execute Chromium/native pixels and transition contracts.
• Run performance, allocation, lifecycle, idle-publication, exact Code OSS, and package gates.

Broader animated paint properties and full CSS Animations/WAAPI remain outside this slice.