Catalog replacement URIs (uri, rewritePrefix, delegate/next catalog) are
now resolved against the catalog's base URI and any in-scope xml:base,
reusing the existing RFC 3986 URIReference resolver. Resolver(_:baseURI:)
supplies the catalog's own location; xml:base on the catalog/group/entry
elements scopes nested entries. An empty base leaves relative URIs as
written, so catalogs without a base behave exactly as before.
The legacy SGML catalog format and group-level prefer override remain.
Part of #86
