Extract prefetch-related fields from __wt_connection_impl (lines 847-856) into a new WT_CONN_PREFETCH structure. This includes prefetch_threads (WT_THREAD_GROUP), prefetch_queue_count, pfqh (the queue itself), prefetch_auto_on, and prefetch_available. Note: prefetch_lock will remain in WT_CONN_LOCKS (from ticket 1.1). Access will be via conn->prefetch.<field>.

Files affected

• src/include/connection.h — Define WT_CONN_PREFETCH structure and replace individual prefetch fields
• src/block/block_read.c — Update prefetch queue operations
• Any files managing prefetch threads or queue

Definition of Done

• New WT_CONN_PREFETCH structure defined in connection.h containing all prefetch fields
• All prefetch fields removed from __wt_connection_impl and replaced with single prefetch member
• All references updated from conn->prefetch_threads to conn->prefetch.threads, etc.
• Code compiles cleanly with no warnings
• All existing tests pass
• Grep confirms no stale references to old prefetch field paths