Follow-up to the mapper retirement (the new docs missed the prior commit's
staging). Adds docs/IMPORT-ADOPTION.md (two-layer import story + stage/crosswalk
guidance), scripts/migration/README.md (dir superseded, points at the API +
loader), and updates the WJ loader README to complete status.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
