resolveFontFamily

Resolve a CSS font-family value to a Compose FontFamily.

Generic family names (sans-serif, serif, monospace, cursive) map to Compose's built-in families. A specific named family (e.g. "Roboto") is left to the platform — we return FontFamily.Default so the platform's default font is used. Custom fonts are out of scope at v1 (caller can override via CompositionLocal in a later step).