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).
Grow the Tier-2 conformance corpus with 13 XPath navigation cases (descendant
and all-element counts, positional and last() predicates, ancestor and
ancestor-or-self axes, following/preceding-sibling, the attribute axis, an
attribute predicate, union, the parent abbreviation, and position()) over a
fixed tree, checked through the validation-driven harness. Own suite.
Part of #105 (Tier 2)
