EE: record why recRAMCopy is sized to MainRam, not ExposedRam

Upstream x86 sizes the snapshot buffer, its guard and its LUT to
Ps2MemSize::ExposedRam, which follows the 128MB devkit mapping; ours uses
MainRam. That looks like a portable one-line divergence and is not: this rec
is MainRam-only throughout — recLutEntries and the recRAM advance in
recReserveRAM, the (MainRam / _64kb) alias mask in recResetRaw that folds all
guest RAM into a 32MB LUT window, and the manual_page / manual_counter
arrays.

Widening the buffer on its own would hand two blocks aliased to the same LUT
entry two distinct snapshots, which is worse than the coverage it buys. Note
the constraint at the allocation so the next reader does not try it.

The real gap — that this rec does not support the 128MB mapping at all —
stands, and needs the LUT, the mask and the manual-page arrays converted
together.

Comment only, no behaviour change. recompiler_tests 1439/1439.

pcsx2/arm64/iR5900-arm64.cpp

@@ -2970,6 +2970,11 @@ static void recReserveRAM()
 	recROM2 = curpos;
 	curpos += (Ps2MemSize::Rom2 / 4);
 
+	// MainRam, deliberately — this whole rec is MainRam-only: recLutEntries and
+	// the recRAM advance above, the (MainRam / _64kb) alias mask in recResetRaw,
+	// and manual_page / manual_counter. Upstream x86 uses ExposedRam throughout
+	// instead (iR5900.cpp:564-577). Widening this buffer alone would desync the
+	// snapshots from a LUT that aliases high RAM back into the low 32 MB.
 	if (recRAMCopy.size() != Ps2MemSize::MainRam)
 		recRAMCopy.resize(Ps2MemSize::MainRam);
 }