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).
A complex type whose simpleContent derives from a list or union simple type now
keeps that variety, so its element content is validated as a list or union rather
than collapsing to an atomic value space. simpleContentType built the content type
with only the resolved base and facets, dropping the variety, so a list-typed
simpleContent accepted any whitespace-separated text and an invalid item (e.g. a
non-int in a list of int) was not caught.
A valid list/union value is unaffected; the change only tightens validation of
content that was wrongly accepted. The W3C XSTS corpus does not exercise this
case, so all four bucket counts are unchanged (vsr=0, isa=67, vir=0, ivi=31); the
gap was surfaced by the libxml2 differential and is covered by a unit test.
