Pure module — no I/O, mirrors the separation established by
order-quote-match.ts.
isExactItemMatch is multiset equality on (normalizeLine(description),
quantity). Empty sets return false (would otherwise vacuously flag
every zero-line order against every zero-line DN).
14 tests cover: exact match, qty diff, extra line on either side,
multiplicity enforcement, the empty-set case, normalization
collapsing whitespace + case, and punctuation-sensitive 'exact'.
Co-Authored-By: Claude <noreply@anthropic.com>
