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 the PureXML.Canonical namespace rendering the C14N form: namespaces
sorted by prefix (default first), attributes by namespace URI then local
name, empty elements expanded to start/end pairs, CDATA folded into escaped
text, and the C14N escaping rules. Inclusive mode renders every in-scope
namespace; exclusive mode renders only visibly-used namespaces plus an
InclusiveNamespaces prefix list. Comments omitted unless requested. For
whole-tree canonicalization C14N 1.0 and 1.1 coincide.
Closes#54Closes#55Closes#56Closes#26
