xs:unique/xs:key/xs:keyref were quadratic over a wide list of targets.
The duplicate check compared each value tuple against every tuple already
seen (and a keyref against every referenced key), and the error-location
helper rescanned the parent's whole child list to position each target.
Key tuples in a Set by their raw string when field equality provably
reduces to a raw string comparison (whitespace-preserving lexical types,
no colon, which is most ids); value-space types (numeric, boolean, date),
whitespace-collapsing types, and QName-bearing values keep the exact
pairwise comparison, so which documents pass is unchanged. Position each
target through a per-parent, per-name index built once.
A 16000-item xs:unique document drops from about 4.0 s to about 0.12 s
(~34x), and the cost is now linear. The full local suite and the W3C
XSTS datatype/identity conformance corpus are unchanged.
