Group @testable imports at the bottom, per public/rules-swift's swiftformat. A
behavior-preserving import-ordering reformat across 252 files (mostly tests).
--trailingclosures was deliberately not adopted (cosmetic, and swiftformat flags
it as caution-worthy).
Add PureXML.Parsing.TextReader (via PureXML.reader), a pull cursor over the
streaming core. read() advances node by node, exposing nodeKind, name,
value, depth, attributes, and isEmptyElement. A childless element is
reported once as an empty element, normalizing <a/> and <a></a>. The
documentType accessor surfaces the DTD read so far as the validation hook.
Closes#33
