A general entity whose replacement text contains markup (directly or
transitively) is reparsed as content at the reference (4.4.2 Included):
the replacement is WFC-validated, cycle-checked, and budgeted, then
pushed back into the reader stream, so its elements become elements in
the tree and the pull-event stream rather than escaped character data.
Character references in internal entity literals now expand at
declaration time for general entities too (4.4.5), making char-ref
markup splice and the Appendix D double escape render as a literal
ampersand; the reference-time WFC validates remaining references
instead of re-expanding. Text-only entities keep the coalescing
single-pass path. Sun valid is 28/28.
Closes#133
