Build `tree_max_depth(node)` over a nested-dict binary tree: - A node is `{"value": N, "left": <node or None>, "right": <node or None>}`. - `None` → depth 0 (empty tree). - Single root → depth 1. - Otherwise → `1 + max(depth(left), depth(right))`. This is LeetCode #104 — the cleanest demonstration of "base case + combine" tree recursion.