Progressive migration

Pressundo supports three non-destructive strategies:

Strategy	Behavior
preserve	Existing legacy storage stays the source of truth. New installations use modern storage.
lazy	Read legacy only when modern is missing, copy the accessed value once, then use modern.
copy	Use modern storage while synchronizing existing legacy storage for rollback/backward compatibility.

There is intentionally no destructive move strategy in Free 1.0.

Field/config mapping can use migrate_from plus optional from_legacy, to_legacy and sanitize callbacks.