Add control primitives, parity docs, and visual snapshots

[seregonwar]

Summary

• add shared control state and accessibility semantics for widgets, integrating Button, Checkbox, RadioButton, and Slider
• document SwiftUI parity and provide a control gallery example demonstrating new interaction states
• add snapshot-based visual tests for control interactions and hover/press/focus/disabled animations

Testing

• cargo test -p strato-widgets --offline button_state_snapshots -- --nocapture
• cargo test -p strato-widgets --offline slider_snapshots -- --nocapture
• cargo build -p control_gallery --offline