Add legacy database structural reference for non-Windows machines
docs/LEGACY_DATABASES.md documents all three source Access databases (every table, column, type, and known data-quality quirk) generated from a live read of the real files, so no Windows/Access driver is needed to understand their structure going forward. New migration/ tooling: catalog_schema.py connects to the real files and walks every table (including excluded scratch tables); render_catalog_md.py renders that into the doc's appendix. Raw output checked in at migration/catalog.json so the doc can be regenerated without touching Access again.
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