Function button 

pub fn button(p: &ButtonProps) -> String

Returns the canonical CSS class string for a button.

Upstream basecoat uses compound classes: btn-{size}-{variant} for non-default sizes, and btn-{variant} for default (no size prefix). Examples:

• Default + Primary → "btn-primary"
• Sm + Outline → "btn-sm-outline"
• Lg + Destructive → "btn-lg-destructive"
• Icon + Ghost → "btn-icon-ghost"