Implement com_stmt_fetch

This is used by some clients (especially the mysql C API) to load result rows after a com_stmt_prepare and com_stmt_execute. It requires storing the result set in server memory. It's currently a blocker for getting sysbench working.

Start by tracing the existing com_stmt_prepare and com_stmt_execute handling to understand where prepared-statement results are represented and retained. Implement the missing com_stmt_fetch flow with server-side result storage, then verify that clients using the MySQL C API can load rows after prepare and execute, including the sysbench use case.

Written by the indexing model from the issue text.