`AssetPath::resolve` and `resolve_embed` now take `&AssetPath`

AssetPath::resolve and AssetPath::resolve_embed no longer accept &str and now take &AssetPath directly. The previous string-based APIs have been renamed to resolve_str and resolve_embed_str.

This change avoids unnecessary string allocation and parsing when an AssetPath is already available. To migrate, pass an AssetPath directly to resolve or resolve_embed; when working with strings, use the corresponding *_str methods instead.