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 7 OASIS catalog cases through the
harness: exact system/public/uri entries, rewriteSystem and rewriteURI prefix
matching, and the unresolved outcomes for unmatched identifiers. All conform.
Part of #105 (Tier 2)
