Three XInclude gaps from the audit:
- accept / accept-language / encoding are surfaced to the loader via a new
XIncludeRequest and a process(_:base:loading:) overload, so a loader can
honor content negotiation. The existing loadingURI form is kept.
- Cycle detection tracks the visited (resolved-URI) set along the current
inclusion chain and raises XIncludeError.cycle, instead of relying only
on the depth backstop; a non-cyclic diamond include is not flagged.
- parse="text" combined with an xpointer is rejected
(XIncludeError.textWithFragment), since a fragment cannot select within
unparsed text.
xpointer range() inclusion still depends on the XPointer range() scheme
(tracked in the XPointer issue).
Part of #87
