refactor: make context flow fields readonly per pr review

Made _contextStates and _transformationMatrices readonly in ContextFlow class
as they are initialized once in constructor and never reassigned.

Addresses PR #477 review comments.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: ooples

src/NestedLearning/ContextFlow.cs

@@ -14,8 +14,8 @@ public class ContextFlow<T> : IContextFlow<T>
 {
     private readonly int _numLevels;
     private readonly int _contextDimension;
-    private Vector<T>[] _contextStates;
-    private Matrix<T>[] _transformationMatrices;
+    private readonly Vector<T>[] _contextStates;
+    private readonly Matrix<T>[] _transformationMatrices;
     private Matrix<T>[] _compressionMatrices;
     private static readonly INumericOperations<T> _numOps = MathHelper.GetNumericOperations<T>();