SaveMigration typedef
A single save-format migration step.
Receives the decoded save file (the full top-level map, including
version, timestamp, metadata and resources) at version N and
returns the equivalent data at version N + 1. The SaveManager sets
version on the returned map itself, so steps don't need to.
Steps may mutate and return data or build a new map.
Implementation
typedef SaveMigration =
Map<String, dynamic> Function(Map<String, dynamic> data);