- Add crates/pdftract-schema-migrate/ workspace member
- Implement migration framework for v1.x schema versions
- MigrationRegistry with version-pair migration functions
- Identity migration for v1.0 -> v1.0
- Validation: rejects major version changes and downgrades
- Convenience API: migrate(), run_migration(), read_json(), write_json()
- Add migrate-schema CLI binary
- --from/--to version arguments
- stdin/stdout or file I/O support
- Auto-detect pretty-print for terminal output
- Full test coverage for migration registry and validation
Closes bf-4w2rt. Verification: notes/bf-4w2rt.md
