fix: correct right-heavy AVL deletion rotations

[tianrking]

Description of Change

Fix the right-heavy rebalancing conditions in AVL deletion. When the right child has balance -1, deletion currently selects a right-left rotation; when its balance is +1, deletion selects a single left rotation. These choices are reversed. For example, inserting 2, 1, 4, 5 and deleting 1 selects a rotation whose required child is absent, while inserting 2, 1, 4, 3 and deleting 1 leaves the tree unbalanced.

Use a single left rotation for right-child balance <= 0 and a right-left rotation for balance > 0. Add self-tests before the existing interactive example: six deletion rotation cases, sixteen deterministic insertion/deletion permutations, duplicate and missing-key operations, and signed key boundaries. Each mutation checks BST ordering, actual versus stored height, AVL balance, and the expected node set/count.

References

Validation on a public fork using standard Ubuntu 24.04 runners:

• Original behavior: two reproducible failures and four passing controls with GNU 13.3, C11, and active assertions; ASan/UBSan confirms the incorrect rotation.
• Exact final commit validation: built-in self-tests and all six standalone cases, ASan/UBSan, full Debug CMake build, and Clang 18 format/static checks. The static-check helper omits the obsolete AnalyzeTemporaryDtors configuration field; the repository's configured checks and warnings-as-errors remain enabled.

The validation workflow and probe are separate from this PR.

Checklist

• Added description of change
• Added tests and example, test must pass
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Searched previous suggestions before making a new one
• I acknowledge that all my contributions will be made under the project's license

Notes: Existing AVL deletion now chooses the correct single or double rotation for a right-heavy subtree.