The WT_CLAYERED_ITERATE_NEXT/PREV flags were set and cleared in many scattered places across the layered cursor code (before nearly every API method, in update_state, reset_cursors, stable reopen, etc.), making the iteration state hard to reason about and easy to break.

Centralize the handling:

• The flags are cleared in one place during enter(), and set in one place - the top-level next/prev - after a successful or prepare-conflicting walk.
• The per-operation decision of whether the parked alternate constituent is reusable is computed once at enter() into op->alternate_positioned. The iteration machinery works off a bool direction and that field, and no longer touches the flags.

Motivation: stop having to remember to clean up the iteration flags in every code path.