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 opt-in DTD processing behind Limits(allowDoctype:), off by default. When on,
DoctypeScanner reads the <!DOCTYPE> internal subset and honors internal
<!ENTITY name "value"> general entities, which EntityDecoder expands recursively
in text and attribute values.
Safety is mandatory and reimplemented from the Expat amplification-accounting
model (no upstream code copied):
- only entity-expanded characters count against a shared budget
(Limits.maxEntityExpansion); overflow raises amplificationLimitExceeded, which
defeats billion-laughs.
- a visiting set detects recursive entities (recursiveEntity).
- undeclared references raise undefinedEntity.
- external entities are never stored or loaded, so XXE stays closed.
The default still rejects <!DOCTYPE>. 8 DTD tests (incl. billion-laughs and the
external-entity refusal); 49 total.
