Minor code review update

[Chuck Walbourn (walbourn)]

Copilot noted the following issue when reviewing the code:

The default move constructor and move assignment are unsafe. 
The allocator owns raw page-list pointers (m_pendingPages,
m_usedPages, m_unusedPages) and scalar counts, which are
copied rather than nulled during a move. The moved-from
destructor then frees the pages, leaving the destination with
dangling pointers; move assignment can also leak the
destination’s existing pages and later double-free the 
source’s pages. Delete the move operations or implement a
custom move that transfers and clears all ownership state.