perf(actions): use cnode and O(1) parent checks in chawathe edit script generator

[HarshK97]

Problem

1. GenerateEditScript was cloning full 12-field ASTNode trees during edit script generation, so it was creating heavy memory allocations and constant double-map lookups between copies and originals.
2. Sibling alignment was running linear slices.Contains(x.Children, dst) checks on every candidate pair, so alignment turned into an $O(K^2)$ scan whenever nodes had large child lists.
3. LCS dynamic programming was allocating 2D slices on every single call, adding constant memory churn during tree traversal.

What Changed

• Replaced full ASTNode tree copies with a small 5-field cnode struct (orig, nodeType, label, parent, children) that holds only what Chawathe needs.
• Switched child alignment parent checks to direct pointer comparisons (dst.Parent == x and src.parent == w), so parent verification drops from $O(K^2)$ to $O(1)$.
• Flattened the 2D DP matrix in lcs() into a single 1D slice buffer, adding fast-paths for $m=1, n=1$ sequences.
• Updated unit tests in internal/actions/action_test.go to test cnode tree copying and child insertion directly.