feat(engine): add parse error limit flag and tree-sitter error unwrapping

[HarshK97]

Problem

1. Parse errors or unparsed macro fragments in Tree-sitter caused ERROR and MISSING nodes to mess up structural AST matching.
2. Hardcoding a fallback to line diff mode on every single syntax error was too restrictive, so even a tiny mistake in one line dropped the entire file out of AST matching.

What Changed

• Added -e, --parse-error-limit (defaulting to 0) to diffm diff, matching how Difftastic caps parse errors before falling back to line diffing.
• Added ParseErrorCount to ASTNode and a recursive countErrorNodes helper to track ERROR and MISSING nodes across the AST.
• Updated cmd/pipeline.go so line diff fallback only triggers when ParseErrorCount goes over the limit.
• Implemented unwrapErrorNode in internal/treesitter/ast.go to pull valid syntax nodes out of ERROR wrappers, so structural AST matching keeps running on valid parts of the file when under the limit.
• Cleaned up AST construction helpers in internal/treesitter/ast.go to simplify node aliasing, byte offset bounds clamping, and error node unwrapping loops.
• Added unit tests in cmd/diff_test.go and updated golden diff baselines for test fixtures with syntax errors.