All explanations

• No error file at all — You will understand why a missing `LAST_FATAL_ERROR.md` file does not by itself mean the device is healthy.
• Why credentials survive a reflash — You will understand how GoSD keeps a device's settings safe across a reflash, and why the identity key is different.
• Why format authorization works this way — You will understand why `ATFS_FORMAT_DRIVES_IF_NOT_ATFS` takes a date, and why the two drives it covers carry different risks.