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).
Deepen the two thin coverage zones (#119) through the validation-driven
harness. Serializer options matrix (10 cases): self-close on/off, CDATA kept
vs as-text, asciiOnly hex character references, round-trip carriage-return
escaping, declaration with standalone, exact pretty-print shape, attribute
escapes, and the Latin-1 output-encoding declaration via the byte path. HTML
depth (8 cases): the adoption agency's furthest-block and deep-misnesting
paths, foster-parented table text (with the implied tbody per the HTML5
algorithm), template content nesting through the document parser, and
raw-text script, implied dt/dd and table-cell closes, and comments through
the fragment parser. All conform; the one initial mismatch was a wrong
expectation in the test (the implied tbody), not a code divergence.
Closes#119
