feat(engine): add zhang-shasha tree edit distance for small subtree recovery

[HarshK97]

Problem

1. Small unmapped subtrees inside matched container pairs relied solely on sibling LCS heuristics (SimpleRecovery), which missed optimal AST node pairings whenever node labels or child ordering shifted slightly.
2. Without fine-grained tree edit distance, recovery on compact subtrees produced unnecessary delete/insert pairs instead of tracking exact node updates and moves.

What Changed

• Implemented the Zhang-Shasha (1989) tree edit distance algorithm in internal/engine/zs.go to compute optimal node mappings on small subtrees.
• Updated container recovery in internal/engine/recovery.go to route subtrees under 20 nodes to Zhang-Shasha recovery before falling back to SimpleRecovery.
• Updated internal/engine/bottom-up.go to call Recover during bottom-up container matching and root unmapped child recovery.
• Added Zhang & Shasha (1989) and Chawathe et al. (1996) citations to README.md under Acknowledgements.
• Updated test fixture baseline reflecting sharper AST alignment.