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).
Resolver(sgml:) parses the legacy SGML catalog format: PUBLIC, SYSTEM,
DELEGATE, CATALOG, BASE, and OVERRIDE entries with -- comment --
delimiters. Replacement URIs resolve against BASE and the supplied
baseURI; OVERRIDE YES/NO maps to the prefer-public fallback. Unknown
keywords are skipped tolerantly. Feeds the same Resolver as the XML
catalog.
Part of #86
