fix: align flat fallback NaN distances with Lance - #5293
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Flat fallback currently assigns cosine distance 1 to zero-norm vectors and can return NaN distances for other metrics, unlike Lance's native search.
Preserve NaN for undefined cosine distances and exclude NaN candidates before local top-k across metrics, while retaining infinite distances. Skip table filtering when there are no NaNs to avoid copying the scanned columns.
Validation: 38 distance-semantics tests passed against PyLance 10.0.0-beta.7, including zero-norm vectors/queries, NaN inputs/queries, and positive/negative infinity preservation. Ruff lint/format, mypy, and diff checks passed.