Gap 1: closeLeaf was only tested closing the first sibling; added test
closing the second sibling to cover the else branch of parent->a.get()==leaf.
The test verifies the correct sibling survives with its signal intact.
Gap 2: closeLeaf was only tested at depth 1 (root's direct children);
added test with depth-2 leaf (in right subtree) to exercise findParent's
recursive search in both subtrees. Tests that the correct leaf is promoted
and remaining signals are preserved.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
