Hover optimizations: fingerprinting + layout measurement cache

[jaytaph]

• At document load, HoverFingerprints::build() scans all stylesheets and records which element types, classes, and IDs are subjects of a :hover rule
• On every mousemove, the engine checks whether the newly hovered element matches any fingerprint before triggering a style recalc. elements with no :hover rules are skipped entirely
• Handles the universal case (*:hover / bare :hover): marks the document as fully hover-sensitive so the fast path is not incorrectly applied
• Wired into the tab worker's hover event handling; the pipeline-cairo example now also passes hover state through