Limit Solarium effect to iOS 26

[vistar941]

Summary

• Use the existing useSolariumEffect availability gate for blur and vertical offset.
• Use the same gate when resolving the default spring animation.
• Keep the existing fade behavior on iOS versions earlier than 26 and on non-iOS platforms.

Why

The iOS 26 availability check was defined but not used. The implementation referenced preferredSolariumEffect directly, so string-based title APIs enabled the Solarium animation on every supported platform.

Impact

The Solarium-style animation is now limited to iOS 26 and later when using a string-based scrollAwareTitle overload. ViewBuilder-based titles and other platforms retain the standard fade transition.

Validation

• swift test
• xcodebuild for generic/platform=iOS Simulator
• xcodebuild for generic/platform=macOS,variant=Mac Catalyst
• xcodebuild for platform=macOS