chore: group pseudo-selector examples into sections

[MatiPl01]

Summary

Organizes the Pseudo Selectors example screen so the individual selectors are grouped together and separated from the edge/test cases, which were previously interleaved. Uses the same flatten grouping the Animated Properties -> Base Properties screen already uses, so the screen stays a single flat list with section headers (no extra drill-in):

• Selectors - :hover, :active, :active-deepest, :focus, :focus-within
• Showcase - Form, Planets
• Test Examples - selectors block render transition, Looping transition + :hover, Per-state transition configs, Arbitrary web selectors

It also flips the "SVG radius" :active example to start large and shrink on press - it started small, which made it a hard press target.

Test plan

Verified on the iOS simulator: the Pseudo Selectors screen renders the three sections in order with their examples as cards underneath, and navigating into each example still works. Typecheck passes.

Stacked on #9879.