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).
An <xs:attribute ref="..."> reference has no name attribute, so attributeUse
returned nil and the referenced attribute was dropped, making it look undeclared
on the instance (nested attributeGroup refs were already flattened, but the
per-attribute reference was not). References now resolve through a new global
attribute index to the declaration's type, with use/default/fixed taken from the
reference site and the name qualified to the target namespace.
XSTS: valid instances rejected 338 to 311. Baseline updated; regression tests
cover a referenced attribute admitted, its type enforced, and an attribute
outside the group still rejected. The attribute-parsing functions move to
XSDAttributeParser.swift to keep XSDParser within the type-body length limit.
Refs #147, #129.
