Make Node.infoset, Node.outcome, Node.player lazy, node-anchored views (#946)

[d-kad]

Closes #946

Description of the changes in this PR

Makes three node properties — Node.infoset, Node.outcome, and Node.player — return
lazy, node-anchored views (NodeInfoset, NodeOutcome, NodePlayer) that resolve on each
access rather than capturing a value at access time. A stored reference now tracks game
mutations and re-resolves to reflect the current tree, consistent with the already-lazy Node.children.

Each view resolves to None (and is falsy) when the relation is absent: a terminal node has
no infoset and no player; same for a node with no attached outcome. At a chance node,
Node.player resolves to the chance player (is_chance is True).

Node.parent remains eager as agreed.

Notes

• The views are hashable (by their resolved object); transitional. No public resolve().
• Infoset.__eq__, Outcome.__eq__, and Player.__eq__ now return NotImplemented on type
  mismatch (was plain False), so a real object and its view compare equal from either side.

[tturocy]

This looks positive, so let's proceed with .outcome and .player as discussed and combine them into one atomic change.