fn migrate_legacy_data_dir(paths: &DataPaths)Expand description
One-time migration: copy channels/<ch>/data/db/ into the new
channels/<ch>/versions/<v>/data/db/ when:
(a) the new versioned db dir has no DB files yet, AND
(b) the old unversioned db dir does exist (pre-Phase-2 install).
Uses copy not move so the old data is preserved for older binaries that may still be installed. Silent on any I/O error — a fresh DB is safer than a partially migrated one blocking launch.