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 the PureXML.Pattern namespace: a compiler for the streamable XPath
subset (names, *, prefix:*, /, //, leading absolute /, trailing @attr) into
a Matcher, with predicates and . / .. rejected. The backward-matching
algorithm tests an element's ancestor-or-self path. Pattern.matches(_:in:)
streams a document through the pull parser and returns matching node paths
in document order from the open-element stack alone, no tree built.
Closes#44Closes#45Closes#22
