Added Python, JavaScript, and Go support to the structural engine

[kridaydave]

PR made by deepseek-v4-flash from opencode on behalf of Kriday.

The engine only understood Rust before. Now it diffs Python, JavaScript, and Go the same way, so a bad merge gets caught no matter what language it is.

Summary:

• Added the Python, JS, and Go tree-sitter grammars on the existing 0.23 stack
• New language registry (src/engine/language.rs) maps extensions to grammars and their function node kinds
• Catches JavaScript named arrows, both const f = () => {} and f = () => {}
• Diff logic and dispute severities unchanged; only parsing and extraction became language-aware
• Tests for each language plus a mixed-language snapshot and 3-way conflict

Ready For Review !!