Extract the core debug-mode fields from _wt_connection_impl into a new WT_CONN_DEBUG structure. Scope is limited to fields with an explicit debug prefix and the disagg address cookie debug state:

• debug_ckpt, debug_ckpt_alloc, debug_ckpt_cnt, debug_log_cnt (protected by debug_log_retention_lock)
• debug_flags + all WT_CONN_DEBUG_* flag definitions
• debug_disagg_address_cookie_upgrade, debug_disagg_address_cookie_optional_field

Fields not included in this ticket: extra_diagnostics_flags, verbose, json_output, timing_stress_flags — these remain on __wt_connection_impl for now.

Access will be via conn->debug.<field>.

Files affected

• src/include/connection.h — Define WT_CONN_DEBUG structure and replace the above fields
• src/conn/conn_debug.c — Update debug configuration
• All files that reference the moved fields

Definition of Done

• New WT_CONN_DEBUG structure defined in connection.h containing only the scoped fields above
• debug_ckpt, debug_ckpt_alloc, debug_ckpt_cnt, debug_log_cnt, debug_flags, debug_disagg_address_cookie_upgrade, debug_disagg_address_cookie_optional_field removed from __wt_connection_impl and placed in WT_CONN_DEBUG
• All references updated (e.g. conn->debug_flags → conn->debug.flags, conn->debug_ckpt → conn->debug.ckpt)
• Code compiles cleanly with no warnings
• All existing tests pass
• Grep confirms no stale references to old field paths