refactor(battle-node): de-magic wire flags and scattered constants

Quality pass from the 2026-06-04 BattleNode review (audit in the outer
repo). All changes are behavior-preserving — identical wire bytes,
verified by the full 1008-test suite staying green.

- Name scattered magic numbers: crypto key/IV lengths, outbound-sequencer
  base, WS receive buffer / EIO ping / SID length, polite-close timeout,
  upgrade-credential keys, battle-id digit math, deterministic-turn spin.
- resultCode = 1 -> (int)ReceiveNodeResultCode.Success across body records.
- Pong "3" -> EngineIoPacketType.Pong; remove dead NoOpBotParticipant.Touch
  (replace with #pragma warning disable CS0067).
- Wire-flag enums, serialized as numbers via JsonNumberEnumConverter:
  turnState -> TurnState{First,Second}, isSelf -> CardOwner{Opponent,Self},
  open -> ChoiceVisibility{Hidden,Open}.
- isOfficial / isInvoke -> bool / bool? via new NumericBoolJsonConverter
  (reads/writes 0/1; TDD'd). Scoped to the BattleNode wire boundary only;
  MatchContext and the HTTP/AI-start path stay int (AI-start uses -1 as a
  sentinel, so it is not boolean).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Wire/NodeCrypto.cs

@@ -10,6 +10,12 @@ namespace SVSim.BattleNode.Wire;
 /// </summary>
 public static class NodeCrypto
 {
+    /// <summary>Length of the ASCII key, in chars (AES-256 = 32 bytes = 32 ASCII chars).</summary>
+    private const int KeyLength = 32;
+
+    /// <summary>IV length, in chars. The node derives the IV from the first half of the key.</summary>
+    private const int IvLength = KeyLength / 2;
+
     /// <summary>
     /// Generate a fresh 32-char key for server-initiated encryption.
     /// Calls <paramref name="randHexDigit"/> 32 times; the result is masked with
@@ -27,20 +33,20 @@ public static class NodeCrypto
     /// </remarks>
     public static string GenerateKey(Func<int> randHexDigit)
     {
-        var sb = new StringBuilder(32);
-        for (var i = 0; i < 32; i++)
+        var sb = new StringBuilder(KeyLength);
+        for (var i = 0; i < KeyLength; i++)
         {
             sb.Append((randHexDigit() & 0xF).ToString("x"));
         }
         var ascii = Encoding.ASCII.GetBytes(sb.ToString());
-        return Convert.ToBase64String(ascii).Substring(0, 32);
+        return Convert.ToBase64String(ascii).Substring(0, KeyLength);
     }
 
     /// <summary>Encrypt: returns key + base64(AES-256-CBC(plain)).</summary>
     public static string EncryptForNode(string plaintext, string key)
     {
-        if (key.Length != 32)
-            throw new ArgumentException($"Key must be exactly 32 chars, got {key.Length}", nameof(key));
+        if (key.Length != KeyLength)
+            throw new ArgumentException($"Key must be exactly {KeyLength} chars, got {key.Length}", nameof(key));
         using var aes = BuildAes(key);
         using var encryptor = aes.CreateEncryptor();
         var plainBytes = Encoding.UTF8.GetBytes(plaintext);
@@ -51,10 +57,10 @@ public static class NodeCrypto
     /// <summary>Decrypt: input[0..32] is key, input[32..] is base64(ciphertext).</summary>
     public static string DecryptForNode(string encrypted)
     {
-        if (encrypted.Length < 32)
-            throw new ArgumentException("Encrypted blob is shorter than the 32-char key prefix", nameof(encrypted));
-        var key = encrypted.Substring(0, 32);
-        var cipherBytes = Convert.FromBase64String(encrypted.Substring(32));
+        if (encrypted.Length < KeyLength)
+            throw new ArgumentException($"Encrypted blob is shorter than the {KeyLength}-char key prefix", nameof(encrypted));
+        var key = encrypted.Substring(0, KeyLength);
+        var cipherBytes = Convert.FromBase64String(encrypted.Substring(KeyLength));
         using var aes = BuildAes(key);
         using var decryptor = aes.CreateDecryptor();
         var plainBytes = decryptor.TransformFinalBlock(cipherBytes, 0, cipherBytes.Length);
@@ -62,9 +68,10 @@ public static class NodeCrypto
     }
 
     /// <summary>
-    /// Configure an AES-256-CBC instance with the node's IV derivation (first 16 chars
-    /// of the key, UTF-8). Callers own disposal. Assumes <paramref name="key"/> is the
-    /// 32-char ASCII key the encrypt / decrypt path has already validated.
+    /// Configure an AES-256-CBC instance with the node's IV derivation (first
+    /// <see cref="IvLength"/> chars of the key, UTF-8). Callers own disposal. Assumes
+    /// <paramref name="key"/> is the <see cref="KeyLength"/>-char ASCII key the encrypt /
+    /// decrypt path has already validated.
     /// </summary>
     private static Aes BuildAes(string key)
     {
@@ -73,7 +80,7 @@ public static class NodeCrypto
         aes.Mode = CipherMode.CBC;
         aes.Padding = PaddingMode.PKCS7;
         aes.Key = Encoding.UTF8.GetBytes(key);
-        aes.IV = Encoding.UTF8.GetBytes(key.Substring(0, 16));
+        aes.IV = Encoding.UTF8.GetBytes(key.Substring(0, IvLength));
         return aes;
     }
 }