During interactive drag, the constraint topology is invariant — only the
dragged part's parameter values change between steps. Previously,
drag_step() called solve() which rebuilt everything from scratch each
frame: new ParamTable, new Expr trees, symbolic differentiation, CSE,
and compilation (~150 ms overhead per frame).
Now pre_drag() builds and caches the system, symbolic Jacobian, compiled
evaluator, half-spaces, and weight vector. drag_step() reuses all cached
artifacts, only updating the dragged part's 7 parameter values before
running Newton-Raphson.
Expected ~1.5-2x speedup on drag step latency (eliminating rebuild
overhead, leaving only the irreducible Newton iteration cost).
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