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).
- cdataAsText emits a CDATA section's content as escaped text instead of
a <![CDATA[ ... ]]> section.
- asciiOnly escapes every non-ASCII character as a numeric character
reference, so the output is pure ASCII (libxml2's character-reference
encoding).
Both apply to the Serializer and the incremental Writer through the
shared Escaping rules.
Closes#91
