Experiment with pwsav container saves

[ATEMVEGETA]

This builds on the virtual RTC/save-state idea with a cleaner single-container save file.

Changes:

• Adds a rom.pwsav container with EEPROM, RTC metadata, and emulator state chunks.
• Keeps compatibility with existing .sav/.rtc/.state files by importing them when no .pwsav exists yet.
• Saves through a temporary file and atomic replace so an interrupted catch-up does not partially advance the save.
• Replays missed RTC midnights through the emulated firmware instead of patching the 7-day history directly.
• Keeps --no-menu support.

Local tests:

• Near-midnight reopen cases passed, including close at 23:59:59, close at 00:00:00, and open-through-midnight.
• Long catch-up test passed for hundreds of missed midnights.
• Forced-kill during loading/catch-up left rom.pwsav byte-for-byte unchanged with no leftover temp file.

This is still an experiment and meant as a starting point for the .pwsav direction you mentioned.