feat: add MMC snapshot/restore support for Atom

[mattgodbolt]

The AtomMMC2 device does not currently have snapshotState/restoreState methods. This means:

• Rewind will reset the MMC to its initial state (losing any file pointer positions, open files, CWD)
• Save-to-file snapshots won't capture MMC state

The PPIA and Branquart bank state are captured (added in #653), but the MMC is not.

What to save

• allfiles array (the virtual SD card filesystem)
• filenum, fildataIndex, fildata (open file state)
• CWD (current working directory)
• configByte, heartbeat
• MMCtoAtom, MCUStatus (register state)
• globalData, globalAmount (command buffer)
• branquartLatch value (already saved in CPU snapshot)

Notes

• The allfiles array contains WFNFile objects with path (string) and data (Uint8Array). The snapshot system's TypedArray-to-base64 handling should cover the data fields.
• For rewind (in-memory snapshots), the WFNFile references survive without serialization. For save-to-file, they need proper serialization.