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).
The IBM section (1094 cases) runs clean to two documented classes:
279 pre-Fifth-Edition character-class cases (P85-89, count asserted
exactly) and 2 PE-in-internal-subset feature cases. Real gaps closed:
a document needs a root element (production 1); entity references in
literals must be Names or character references; WFC: No Recursion
checked at declaration over the general/parameter reference graph;
undeclared PE references split into WFC (standalone, or no external
declarations possible) versus deferred validity finding, also for
attribute-default references; refused external PEs are neither; and
groups split across PE replacements report VC: Proper Group/PE
Nesting. Entity machinery split to DoctypeScannerEntities for caps.
Closes#125
