[WIP] Review ChromeCdpSnapshotService implementation

[Copilot]

• Remove dead code: unused private method computeVisibility in ChromeCdpDomService (duplicate of visibilityStyleCheck local function and calculateVisibility)
• Remove dead code: unused private method computeInteractivity in ChromeCdpDomService (duplicate of calculateInteractivity)
• Remove dead code: unused private method calculateVisibility in ChromeCdpDomService (the merge function uses visibilityStyleCheck + isElementVisibleAccordingToAllParents instead)
• Hoist HighlightManager to a lazy class-level property instead of creating a new instance in each highlight method
• Hoist ClickableElementDetector to a class-level property instead of creating a new instance in calculateInteractivity
• Rename calculateScalability → calculateScrollability to match its purpose and the isScrollable field it populates
• Build and test changes
• Run code review
• Run CodeQL

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