Function resolve_line_height 

pub fn resolve_line_height(spec: Option<LineHeightSpec>, size_px: f32) -> f32

Resolve a possibly-absent CSS line-height against size_px, falling back to DEFAULT_LINE_HEIGHT_MULTIPLIER (line-height: normal) when no value was authored. The single fallback-consumption point every line-height-aware call site outside this module should route through, rather than re-deriving size_px * 1.2 locally.