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 14 DTD content-model cases (sequence
order/completeness, choice exclusivity, the *, +, ? occurrence indicators,
#PCDATA, mixed content, EMPTY, and ANY) validated against the internal subset
and checked through the validation-driven harness. Own suite.
Part of #105 (Tier 2)
