Consider requiring two mutation method for all `DecisionNode`s

We should consider requiring every decision to implement the following methods
```c++
class ArrayDecisionNode : public ArrayNode, public DecisionNode {
public:
// override the current state with a new one (specified as a flattened array).
void assign(State& state, std::span values) const = 0;

// override specific indices of the array
void set_values(State& state, std::span> values) const = 0;
};
```
the responsibility would be on the caller to ensure the given state/changes result in a valid state for the decision, but each implementation should `assert(...)` for correctness.