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 prefer attribute on a <group> (or nested <catalog>) overrides the
catalog-wide preference for the <public> entries within it, inherited
through nested groups. The resolver records each public entry's effective
preference and consults it when falling back from an unmatched system
identifier, so two entries in one catalog can prefer differently. A system
match still always wins.
