A relative system identifier declared inside an external entity
resolves against that entity's own URI (RFC 3986, the C14N resolver):
ExternalID gains base and resolvedSystemID, the scanner threads the
current entity's URI through external-subset and PE scans, and
resolvers receive the merged identifier. Clears eduni E18; the eduni
baseline now holds only the grapheme-lexing class (#135).
Closes#138
Add internal parameter entities (parsed, stored, and expanded within the
DTD, including bare %name; references that inject markup declarations,
bounded by depth and the expansion budget). Record external general
entities and the external subset identifier, and load them only through an
injected EntityResolver (a struct of closures). The default refusing
resolver fetches nothing, so XXE stays closed: an external entity fails as
undefined and the external subset is left unread. Internal declarations win
over the external subset.
Closes#29
