Clear cached number value when XString is set()

[BilawalAhmed0900]

Fixed an issue when old cache number value is used when XString is set()

[djberg96]

I threw AI at this, using your code originally. It came up with this instead: djberg96#2 instead.

Comments from AI:

The PR branch now uses a proper validity flag for the cached number value:

Added a mutable bool m_cachedNumberValid; to the header
All constructors initialize this flag to false
num() checks the flag instead of comparing against 0.0, computes and updates the value/flag as needed
ClearCachedNumber() simply sets the flag to false
The copy constructor preserves the flag's value
Your cache invalidation is now robust and correct for all possible numeric string cases!