Version the save-file format and add compatibility coverage

[JakenHerman]

Summary

The save-file JSON structure is not explicitly versioned today. Compatibility currently depends on serde defaults and ad hoc field additions.

Why this matters

The project already ships save/load and replay as core features. Future schema changes will be much safer if saves declare a format version and the loader has explicit compatibility behavior.

Current examples

• SaveFile includes fields like advanced_stats, game, and snapshots, but no format_version.
• Older saves are partly tolerated through #[serde(default)], but there is no formal migration contract.

Acceptance criteria

• Save files include an explicit format version.
• The loader documents how it handles supported older versions and unsupported future versions.
• If migration is needed, implement a minimal migration path or a clear error for unsupported versions.
• Add tests covering at least one older-format fixture or synthetic JSON payload.
• README or maintainer docs briefly note the compatibility strategy.

Notes

This issue is about making save compatibility intentional, not about guaranteeing indefinite backward compatibility forever.