fix(battle-node): clarify NodeCrypto.GenerateKey contract + add fixed-vector regression test

Replace inaccurate GenerateKey docstring (it claimed to port Cryptographer.generateKeyString
directly but the input shape differs: server uses one hex digit per call, client uses
Random.Next(0,65535) per call). New doc is honest about the difference and explains why
it's safe. Add EncryptForNode_FixedVector_ProducesStableOutput: a pinned AES-CBC vector
that catches encoding/IV/padding regressions that would slip past the roundtrip test.

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

SVSim.UnitTests/BattleNode/Wire/NodeCryptoTests.cs

@@ -41,4 +41,18 @@ public class NodeCryptoTests
     {
         Assert.Throws<ArgumentException>(() => NodeCrypto.DecryptForNode("tooshort"));
     }
+
+    [Test]
+    public void EncryptForNode_FixedVector_ProducesStableOutput()
+    {
+        // Pinned wire-format regression: any change to encoding/padding/IV derivation
+        // that drifts in both directions would still pass the roundtrip test but break
+        // this hardcoded vector — and break interop with the real client.
+        const string plaintext = "hello, node!";
+        const string key = "01234567890123456789012345678901";
+        const string expected = "012345678901234567890123456789015mEezM5MgR7UUEkmx5OzPQ==";
+
+        Assert.That(NodeCrypto.EncryptForNode(plaintext, key), Is.EqualTo(expected));
+        Assert.That(NodeCrypto.DecryptForNode(expected), Is.EqualTo(plaintext));
+    }
 }