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).
An abstract type cannot be the type of an instance element (cvc-elt.4.3.2), so an
instance xsi:type naming an abstract type is invalid as a substitution even when
the element's declared type would otherwise admit it (typeDef01101, xsi:type on an
anyType-declared element). The validator only checked for an abstract DECLARED
type with no xsi:type; abstractXsiTypeError now also rejects an abstract xsi:type
substitute (matched against the namespaced abstract-type set). A concrete type is
unaffected.
invalid-instances-accepted 34 to 32, no false positive (valid-instances-rejected
and valid-schemas-rejected held at 0; a valid instance never names an abstract
type via xsi:type), no other bucket moved.
