Fix SMS/GG debugger step-back non-determinism bug

[CharlesVanEeckhout]

i had originally found this bug on mesen 2.1.1 on 2025-10-20
heres what i had written back then on the snesdev discord:

i have found a non-determinism bug with the step-back function of the mesen debugger
i have coded a homebrew rom to isolate and demonstrate this bug
stepback.gg.zip

steps to reproduce:

1. place breakpoint at game gear memory 0x00B2 (instruction should be ld b, a)
2. continue execution until the breakpoint. notice that A register is $55
3. continue execution past the breakpoint. this $55 is used to make the background dark gray.
4. reset the rom and run rom until the breakpoint
5. this time, use the step-back function (shift+f10) until pc is 0x00a9 (this should be the first of eight nop)
6. continue execution until the breakpoint again. notice that A register is now $AA
7. continue execution past the breakpoint. this $AA is used to make the background light gray.

this is a bug because the value of A register at step 2 and the value of A register at step 6 should be the same value

root cause for the bug:

in Core/SMS/SmsVdp.cpp, some calls to SmsVdp::ProcessVramAccess only occur when _memAccess[cyc] == SmsVdpMemAccess::None
going forward normally, the _memAccess[cyc] would be SmsVdpMemAccess::None, and then inside SmsVdp::ProcessVramAccess, it would be set to SmsVdpMemAccess::CpuSlot, and would eventually get cleared when the scanline ends
but if instead of letting the scanline end, i step-back to before the SmsVdp::ProcessVramAccess call, since _memAccess is not serialized, _memAccess[cyc] remains SmsVdpMemAccess::CpuSlot instead of returning to SmsVdpMemAccess::None as would be expected
this makes the call to SmsVdp::ProcessVramAccess fail to occur as it had the first time around, because _memAccess[cyc] == SmsVdpMemAccess::None is no longer true
because the vram access now fails to occur, stale values remain in the _state.VramBuffer for longer than they should (this is the $AA in my example)

my solution:

i serialized the _memAccess array
this way, stepping-back restores the previous values of _memAccess correctly, thus allowing all SmsVdp::ProcessVramAccess calls that depend on _memAccess to remain consistent, whether or not step-back is used