fix(battle-node): preserve long type on numeric array elements in FromJson

Root cause for the lingering mulligan failure: the inline conditional
expression in MsgEnvelope.ToObject

    JsonValueKind.Number => el.TryGetInt64(out var l) ? l : el.GetDouble(),

unified its branches to the common implicit-convertible type. long→double
is implicit, so both branches collapsed to double and the integer value
silently widened. Inside an array (idxList:[2]), each element came back
as boxed double; OfType<long> in ExtractIdxList then filtered every
entry out, so swapIndices arrived empty and BuildSwapResponse echoed
the unchanged hand — exactly the diff-against-Deal mismatch the client
flagged as "Card swap failed: AbandonCards[2]/DrawCards[]".

Extract a ParseNumber helper that returns object explicitly so each
branch boxes its own runtime type. Also harden ExtractIdxList to accept
any boxed numeric type (long/int/double/decimal/string) so a future
JSON-parser drift can't silently regress this path again.

Two regression tests:
- FromJson_NumericArray_PreservesLongTypeOnEachElement: confirms the
  fix at the JSON-parse layer with a hardcoded "{\"idxList\":[2,3]}".
- Swap_WithIdxListContainingTwo_ProducesHandWithFreshIdxAtPosition1:
  exercises the dispatch end-to-end with a Body holding a real boxed
  long; asserts position 1 of the response hand is the fresh deck idx 4.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.BattleNode/Protocol/MsgEnvelope.cs

@@ -89,7 +89,14 @@ public sealed record MsgEnvelope(
     private static object? ToObject(JsonElement el) => el.ValueKind switch
     {
         JsonValueKind.String => el.GetString(),
-        JsonValueKind.Number => el.TryGetInt64(out var l) ? l : el.GetDouble(),
+        // Extracted to a helper because writing the conditional inline as
+        //   el.TryGetInt64(out var l) ? l : el.GetDouble()
+        // unifies the conditional's branches to the common implicit-convertible type. long→double
+        // is implicit; so the result type collapses to double and the long value silently widens.
+        // Downstream OfType<long> filters then drop the (now boxed-double) entries, which broke
+        // the mulligan idxList extraction. Separate method returns object explicitly so each
+        // branch boxes its own runtime type.
+        JsonValueKind.Number => ParseNumber(el),
         JsonValueKind.True => true,
         JsonValueKind.False => false,
         JsonValueKind.Null => null,
@@ -97,4 +104,10 @@ public sealed record MsgEnvelope(
         JsonValueKind.Object => el.EnumerateObject().ToDictionary(p => p.Name, p => ToObject(p.Value)),
         _ => el.GetRawText(),
     };
+
+    private static object ParseNumber(JsonElement el)
+    {
+        if (el.TryGetInt64(out var l)) return l;
+        return el.GetDouble();
+    }
 }

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -266,8 +266,26 @@ public sealed class BattleSession
 
     private static IReadOnlyList<long> ExtractIdxList(MsgEnvelope env)
     {
-        if (env.Body.TryGetValue("idxList", out var raw) && raw is List<object?> lst)
+        // Defensive: accept any IEnumerable carrying any numeric boxing (long/int/double/decimal/
-            return lst.OfType<long>().ToList();
+        // string). MsgEnvelope.FromJson should box small ints as long, but a parser quirk
+        // anywhere upstream could yield a different boxed type and OfType<long> would silently
+        // drop the entries — that broke the v1 mulligan during smoke.
+        if (env.Body.TryGetValue("idxList", out var raw) && raw is System.Collections.IEnumerable seq && raw is not string)
+        {
+            var result = new List<long>();
+            foreach (var item in seq)
+            {
+                switch (item)
+                {
+                    case long l: result.Add(l); break;
+                    case int i: result.Add(i); break;
+                    case double d: result.Add((long)d); break;
+                    case decimal m: result.Add((long)m); break;
+                    case string s when long.TryParse(s, out var p): result.Add(p); break;
+                }
+            }
+            return result;
+        }
         return Array.Empty<long>();
     }
 

SVSim.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs

@@ -7,6 +7,26 @@ namespace SVSim.UnitTests.BattleNode.Protocol;
 [TestFixture]
 public class MsgEnvelopeTests
 {
+    [Test]
+    public void FromJson_NumericArray_PreservesLongTypeOnEachElement()
+    {
+        // Regression for the v1 mulligan bug: idxList:[2] from the wire was decoded as a list
+        // containing a boxed double (2.0) instead of a boxed long (2L). The conditional expression
+        // `el.TryGetInt64(out var l) ? l : el.GetDouble()` unified its branches to the common
+        // implicit type (double) and silently widened the long. Downstream OfType<long> filtered
+        // every entry out, so swapIndices arrived empty and the server echoed the unchanged hand.
+        const string json = "{\"uri\":\"Swap\",\"viewerId\":1,\"uuid\":\"u\",\"try\":0,\"cat\":1,\"idxList\":[2,3]}";
+
+        var env = MsgEnvelope.FromJson(json);
+
+        var idxList = (List<object?>)env.Body["idxList"]!;
+        Assert.That(idxList.Count, Is.EqualTo(2));
+        Assert.That(idxList[0], Is.TypeOf<long>(), "idxList[0] must be boxed long, not double.");
+        Assert.That(idxList[0], Is.EqualTo(2L));
+        Assert.That(idxList[1], Is.TypeOf<long>());
+        Assert.That(idxList[1], Is.EqualTo(3L));
+    }
+
     [Test]
     public void Roundtrip_PreservesEnvelopeAndBody()
     {

SVSim.UnitTests/BattleNode/Sessions/BattleSessionDispatchTests.cs

@@ -51,6 +51,27 @@ public class BattleSessionDispatchTests
         Assert.That(s.Phase, Is.EqualTo(BattleSessionPhase.AwaitingSwap));
     }
 
+    [Test]
+    public void Swap_WithIdxListContainingTwo_ProducesHandWithFreshIdxAtPosition1()
+    {
+        var s = NewSession();
+        s.ComputeResponses(NewEnvelope(NetworkBattleUri.InitNetwork));
+        s.ComputeResponses(NewEnvelope(NetworkBattleUri.InitBattle));
+        s.ComputeResponses(NewEnvelope(NetworkBattleUri.Loaded));
+        var swapEnv = NewEnvelope(NetworkBattleUri.Swap);
+        // Simulate the client's Swap{idxList:[2]}: the dict shape produced by MsgEnvelope.FromJson
+        // (a List<object?> of boxed long values).
+        swapEnv.Body["idxList"] = new List<object?> { 2L };
+
+        var responses = s.ComputeResponses(swapEnv);
+
+        var swapBody = responses[0].Envelope.Body;
+        var self = (List<object?>)swapBody["self"]!;
+        Assert.That(((Dictionary<string, object?>)self[0]!)["idx"], Is.EqualTo(1));
+        Assert.That(((Dictionary<string, object?>)self[1]!)["idx"], Is.EqualTo(4));  // swapped — fresh deck idx
+        Assert.That(((Dictionary<string, object?>)self[2]!)["idx"], Is.EqualTo(3));
+    }
+
     [Test]
     public void Swap_PushesSwapResponseThenReady_TransitionsToAfterReady()
     {