a352da53133bfa461c982d83483a8fdb528a8f49  [Image Redux] Update cross-fade() to match CSS Image 4

Classified as not security-relevant.
Triage note: CSS parser/type-system rework of cross-fade(): introduces CSS::Crossfade/WebkitCrossfade/ImageOrNone value types, splits consumeWebkitCrossfade from spec consumeCrossfade (multi-component), and stores a Variant in CSSCrossfadeValue. Feature/refactor with no memory-safety bug.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+2.59), clamp_saturate (+1.8), msg_noise (-2.5), area:Source/WebCore/css (+2.0), large_churn (-2.0)