feat: add Phase 2 migration functions for column type and blob markers

Add migrate_columns() and supporting functions for Phase 2 of the
0.14.6 → 2.0 migration:

- analyze_columns(): Identify columns needing type labels
- migrate_columns(): Add core type markers to column comments
- NATIVE_TO_CORE_TYPE mapping for type conversion
- Support for bool/datetime special cases
- Dry-run mode for previewing changes

Also adds placeholder stubs for Phase 3-4 migration functions:
- migrate_external(): For external storage migration
- migrate_filepath(): For filepath attribute migration
- finalize_migration(): For Phase 4 finalization

These functions implement the migration guide documented in
datajoint-docs/src/how-to/migrate-from-0x.md.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko