Save migration and persistence

Status: in progress.

Goal

Protect progress and settings when loading, saving and migrating old data.

To do

• [x] Align old-save detection and reading and reject unreadable migration data without creating an empty replacement.
• [x] Check encrypted saves and example migration with disposable Linux files; simulate both Windows path choices.
• [x] Check a two-record example merge, the new-save priority and corrupt-old-file protection in an isolated Linux profile. The checks reported passing; Unity crashed on shutdown.
• [ ] Check actual Windows migration with representative disposable historical files, both old locations and applicable merge cases.
• [ ] Check settings and keybinding persistence and Windows/Steam profile selection.
• [ ] Decide how to avoid counting duplicate old saves twice and review recovery if a player-save write is interrupted. Keep existing formats readable.

See save tests. Keep original saves and never use real profiles for automated checks.