Perf: restore O(N+M) item filtering in Lookup.GetItems#13688

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JanProvaznik:dev/janpro/lookup-getitems-perf-fix

Commits on May 5, 2026

Perf: restore O(N+M) item filtering in Lookup.GetItems

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Address review: convert assertions to Shouldly, add no-op-removes and adds-path tests

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