[Tree unification 5/9] Validation-based pruning on the kernel Tree

[jeongyoonlee]

Reimplement validation-based pruning (pruneTree) on the kernel Tree.

Depends on Issue 1 (scaffold); best sequenced after Issue 4 (honesty) since both operate as post-build passes over the grown tree.

Scope

• Bottom-up merge-by-validation-gain pass over the kernel Tree node array: for each internal node whose children are both leaves, if merging them does not reduce the uplift gain on the validation set, collapse them into a leaf. Repeat until no beneficial merge remains.
• Replaces the legacy recursive pruneTree.

Verify

• Pruning parity vs legacy on a fixed seed (same validation split, same criterion).

Reusable templates

• Legacy: uplift.pyx pruneTree (~629–784).
• Kernel Tree node array structure (children_left / children_right / value).

Part of #945.