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 10 cases through the harness:
XInclude substitution, parse=text inclusion with escaping, xi:fallback on a
missing href, and relative href resolution against the base; XPointer
shorthand ids, element() navigation from an id and from the root, and
xpointer() including the classic per-step predicate semantic (//para[1]
selects every parent-first para; (//para)[1] the document-first), which the
corpus now locks explicitly.
Part of #105 (Tier 2)
