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 xsi:type naming an undeclared type is now a located error in both the
tree validator (validateChild) and the streaming driver, instead of a silent
fallback to the declared type. typeReference chains resolve through one
shared resolver (resolveReference, with a visited set), replacing the inline
recursion, the 64-hop cap, and the completions guard: a circular chain now
reports 'circular type reference' instead of silently truncating, and the
streaming shallow check no longer recurses without bound on a cyclic table
(a latent crash). Tests cover unknown xsi:type (tree + streaming agree),
the circular chain (tree + shallow), the resolver's three outcomes, and a
conformance-harness case pair.
Closes#110
