Description.

The goal is to introduce a new method, GameNode::IsStrategyReachable(), to determine if a given node in an extensive tree form can be reached by any pure strategy profile. A node is reachable if there exists at least one pure strategy profile where the resulting path of play passes through that node. A pure strategy requires the player to choose the same action at every member node of that information set. In games with absent-mindedness, a path of play may visit the same information set multiple times, making some of the ancestors of its member nodes unreachable.

The existing GameTreeRep::BuildInfosetParents() method already handles the logic for absent-minded revisiting information sets. This traversal will be leveraged to collect the set of reachable decision nodes.

Implementation Plan.

1. In GameTreeRep, add member:
   mutable std::set<GameNodeRep *> m_reachableNodes;
2. Define the public API on the node, consistent with existing properties, e.g., IsTerminal() and IsSubgameRoot():
   In GameNodeRep, add the method bool IsStrategyReachable() const;
3. Modify GameTreeRep::BuildInfosetParents() to populate the m_reachableNodes set.
4. Implement GameNodeRep::IsStrategyReachable() to trigger computation if needed and perform the check.
5. Python API: Add a read-only property Node.is_strategy_reachable.