Enum validation#105
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Error when an `enum` column contains a non-null value outside its declared `values` (issue #72). Parses the allowed values into the model (list items or map keys), builds a canonical set, and checks membership during the data scan via the existing ColumnCheck pipeline. The footer can't settle set membership, so the meta check is always inconclusive for enum columns and defers to the scan.
Enums are almost always dictionary-encoded, so a column chunk's distinct values sit in one small dictionary page. When every dictionary entry is in the allowed set and every data page draws from the dictionary, the column conforms without scanning its values; any doubt falls back to the full scan (which also reports offending rows). ~500x faster on a conforming 10M-row enum column (0.004s vs 1.5s). The parquet-rs writer omits page_encoding_stats, so the data-page check uses those stats when present and otherwise inspects each page's encoding directly (cheap: index pages are tiny and never decoded here).
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Validate that no enum values are outside of the specified set. Has a fast pass where that's true, and then a slower pass to report locations of invalid values.
Fixes #72