Draft: Park MMX password saves sidecar rework

[mstan]

This parks the password-save/SRAM rework instead of shipping it in the next release.

Known issues / reasons this is draft:

• The originally shipped implementation exposed synthetic cartridge SRAM to Mega Man X, a no-SRAM LoROM, changing guest-visible cart mapping and savestate layout.
• Recent reports (Every 2nd jump gets cut off #30, Drops are disappearing when they hit the ground #31) indicate likely gameplay regressions: alternating jump cutoff and disappearing item drops.
• Old local v4 savestates can still fail to load and may hang after a failed load path.
• The sidecar rewrite avoids guest-visible SRAM in focused debug checks, but it has not had enough gameplay validation to ship.

Validation done on the parked branch before parking:

• Fixed trace build reported guest SRAM size 0 via debug read_sram.
• Headless dummy SDL/audio scripted two-jump run completed from a valid v5 state.
• Focused display/widescreen tests passed when run directly.

Do not merge until the gameplay reports are reproduced and the save/update migration behavior is fully covered.