rebase v4.
Early v2/v3 only fixed the page-cross judgment condition, but still retained duplicated
EndPos calculation logic. v4 refactors ReserveXLogSwitch to compute standard EndPos
once and reuse it, eliminating redundant code fundamentally.
All behavior remains identical to v2/v3 after fix; v4 just unifies the calculation
point without changing runtime logic.
ZizhuanLiu X-MAN
44973863@qq.com
