C#: Add StructuralEqualityComparator to SDK

[knutwannheden]

Summary

• Add StructuralEqualityComparator — a public static class for comparing two J (LST) nodes for structural equality, ignoring whitespace, formatting, node IDs, and type attribution
• Consolidate shared reflection helpers (HasNullSafe, IsRightPadded, IsLeftPadded, IsContainer) from PatternMatchingComparator into TreeHelper, eliminating duplication
• Cache PropertyInfo lookups in TreeHelper.UnwrapPadded and GetContainerElements for better hot-path performance

Motivation

Recipe authors in recipes-csharp need to compare AST subtrees for structural equality (e.g., checking if both sides of x ?? x are identical, or if if/else branches match). The existing PatternMatchingComparator handles this internally but is internal and carries capture/placeholder/variadic overhead not needed for simple equality checks.

Usage

// Instead of fragile string comparison:
// binary.Left.ToString().Trim() == binary.Right.ToString().Trim()

// Use structural equality:
StructuralEqualityComparator.AreStructurallyEqual(binary.Left, binary.Right)

Test plan

• 20 xUnit tests covering: identical/different literals, binary expressions, operators, identifiers, whitespace insensitivity, nested expressions, method invocations, string literals, same reference, boolean/null literals, different argument counts, NullSafe vs regular access
• All existing C# module tests pass (no regressions)