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).
Add a canonical11 preset / mergeInheritedBase option. On a document subset,
the omitted-ancestor xml:base chain is merged into the apex by RFC 3986
reference resolution (vs 1.0 nearest-wins), the apex's own relative xml:base
is resolved against it, and xml:id is no longer inherited; xml:lang/xml:space
still inherit nearest. A self-contained RFC 3986 resolver (parse, path merge,
dot-segment removal) backs the merge, with tests covering the 1.1-vs-1.0
difference, dot segments, xml:id, and lang/space.
Part of #105 (Tier 3)
