GPUI spike: rangular button + @if on native renderer

[Interchouette]

Parent

#37

Work

1. Minimal GPUI app loading one rangular-generated component.
2. Template: static text, one @if, one click handler.
3. Compare output with Leptos fixture for same template.

Acceptance

cargo run -p demo-gpui (or equivalent) shows working UI; fixture or snapshot test.