feat: add CalcFunctionFactory for calculator operations (#13)

Callers pick an operation via the CalcOperation enum instead of
instantiating implementation classes directly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/main/java/com/example/util/CalcFunctionFactory.java

@@ -0,0 +1,20 @@
+package com.example.util;
+
+public final class CalcFunctionFactory {
+
+    private CalcFunctionFactory() {
+    }
+
+    public static CalcFunction create(CalcOperation operation) {
+        if (operation == null) {
+            throw new IllegalArgumentException("Operation must not be null");
+        }
+        return switch (operation) {
+            case ADD -> new AddFunction();
+            case SUBTRACT -> new SubtractFunction();
+            case MULTIPLY -> new MultiplyFunction();
+            case DIVIDE -> new DivideFunction();
+            case POWER -> new PowerFunction();
+        };
+    }
+}