The interactive drag section described the original naive implementation
(re-solve from scratch each step) and called the caching layer a
'planned future optimization'. In reality _DragCache is fully
implemented: pre_drag() builds the system, Jacobian, and compiled
evaluator once, and drag_step() reuses them.
Update code snippets, add _DragCache field table, and document the
single_equation_pass exclusion from the drag path.
