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).
Extract PathKey.steps(forChildNames:) as the single path construction behind
the validator walk, the XSD content validator, and the identity validator,
replacing three identical implementations, so located errors can never
disagree on a path. Add two differentials locking the duplicated engines
together: equivalent DTD and XSD content models (sequence, choice, *, +, ?,
nested choice) must agree on every child sequence across the positional
matcher and the Thompson NFA; and the pull (EventReader) and push
(PushScanner) tokenizers must yield the same event stream for the same
document, even with the push side fed one character at a time (adjacent text
events coalesced, since chunk boundaries legitimately split text). Both
pairs agree on everything today; the differentials guard them.
Closes#115
