feat(battle-node): port AES-256-CBC encryptForNode/decryptForNode codec

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.UnitTests/BattleNode/Wire/NodeCryptoTests.cs

@@ -0,0 +1,44 @@
+using NUnit.Framework;
+using SVSim.BattleNode.Wire;
+
+namespace SVSim.UnitTests.BattleNode.Wire;
+
+[TestFixture]
+public class NodeCryptoTests
+{
+    [Test]
+    public void EncryptThenDecrypt_RoundTripsArbitraryString()
+    {
+        const string plaintext = "{\"uri\":\"InitNetwork\",\"viewerId\":906243102,\"try\":0,\"cat\":99}";
+        const string key = "Y2FmZWJhYmU3ZmY3ZmY3ZmY3ZmY3ZmY3"; // 32 chars
+
+        var encrypted = NodeCrypto.EncryptForNode(plaintext, key);
+        var decrypted = NodeCrypto.DecryptForNode(encrypted);
+
+        Assert.That(decrypted, Is.EqualTo(plaintext));
+    }
+
+    [Test]
+    public void GenerateKey_WithDeterministicSource_ProducesStable32CharOutput()
+    {
+        var seq = 0;
+        string key = NodeCrypto.GenerateKey(() => seq++ % 16);
+
+        Assert.That(key.Length, Is.EqualTo(32));
+        // Two calls with the same source produce the same output.
+        seq = 0;
+        Assert.That(NodeCrypto.GenerateKey(() => seq++ % 16), Is.EqualTo(key));
+    }
+
+    [Test]
+    public void EncryptForNode_NonStandardKeyLength_Throws()
+    {
+        Assert.Throws<ArgumentException>(() => NodeCrypto.EncryptForNode("x", "tooshort"));
+    }
+
+    [Test]
+    public void DecryptForNode_TooShortInput_Throws()
+    {
+        Assert.Throws<ArgumentException>(() => NodeCrypto.DecryptForNode("tooshort"));
+    }
+}