Add dedicated metadata filter push-down path through SupportsReadingMetadata.
Metadata predicates are classified separately from physical predicates and
pushed via applyMetadataFilters() with a dedicated MetadataFilterResult type.

MetadataFilterPushDownSpec stores a pre-translated predicateRowType using
metadata key names (not SQL aliases), making the serialized spec self-contained
without needing a column-to-key mapping.

Generated-by: Claude Code (claude-opus-4-6)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>