feat(arena-tk2): split do_matching success into 3007 owner / 3004 joiner

Mirrors prod's TK2 wire flow: the first arriver (parked, picks up cached
pair on a later poll) gets matching_state 3007 (SUCCEEDED_OWNER); the
second arriver (whose poll triggered the pair) gets 3004 (SUCCEEDED).

Observationally inert in the public matching code path today — the
client's Matching class writes isOwner from the response into a field
that nothing in TK2/ranked reads. Matching_Room (private rooms) DOES
read it but from a separate code path that doesn't consult our response.
We send the split anyway for prod fidelity and to leave room for future
flows (rematch UI, etc.) that might start consuming it.

TryPairAsync now returns PairUpResult(Match, IsOwner) instead of bare
PendingMatch?, so the controller can decide owner vs joiner without
re-deriving it.

Also documents on DoMatchingResponseDto why we omit prod's `room_id`
field (not in the client's DoMatchingDetail model; private-room flows
get their room id from a different API and don't consult this response).

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

SVSim.EmulatedEntrypoint/Matching/IMatchingPairUpService.cs

@@ -11,10 +11,28 @@ public interface IMatchingPairUpService
 {
     /// <summary>
     /// Try to pair the calling viewer with an already-waiting partner.
-    /// Returns the resolved <see cref="PendingMatch"/> when a partner was found
+    /// Returns the resolved <see cref="PairUpResult"/> when a partner was found
     /// (either this call paired with a waiter, or a previous pairing's result is
     /// still cached for this viewer). Returns null if this viewer is the first
-    /// arriver and should be parked (caller returns 3001 RETRY).
+    /// arriver and should be parked (caller returns 3002 RETRY).
     /// </summary>
-    Task<PendingMatch?> TryPairAsync(string mode, BattlePlayer player, CancellationToken ct);
+    Task<PairUpResult?> TryPairAsync(string mode, BattlePlayer player, CancellationToken ct);
 }
+
+/// <summary>
+/// A resolved pair-up for a single caller.
+/// <para>
+/// <see cref="IsOwner"/> distinguishes the "original waiter" (first arriver, whose
+/// cached result is being consumed on a follow-up poll — maps to wire matching_state
+/// 3007 = RC_BATTLE_MATCHING_SUCCEEDED_OWNER) from the "joiner" (second arriver,
+/// whose poll triggered the pair — maps to 3004 = RC_BATTLE_MATCHING_SUCCEEDED).
+/// </para>
+/// <para>
+/// The split mirrors prod's TK2 wire flow (waiter sees 3007, joiner sees 3004) but
+/// is observationally inert in the public-matching code path: the client's
+/// <c>Matching</c> class writes <c>isOwner</c> from the response into a field that
+/// nothing else in TK2/ranked reads. We send the split anyway for prod fidelity in
+/// case a future flow (rematch UI, private rooms grafted on top) starts consuming it.
+/// </para>
+/// </summary>
+public sealed record PairUpResult(PendingMatch Match, bool IsOwner);

SVSim.EmulatedEntrypoint/Matching/InProcessPairUp.cs

@@ -18,25 +18,27 @@ public sealed class InProcessPairUp : IMatchingPairUpService
         _bridge = bridge;
     }
 
-    public Task<PendingMatch?> TryPairAsync(string mode, BattlePlayer player, CancellationToken ct)
+    public Task<PairUpResult?> TryPairAsync(string mode, BattlePlayer player, CancellationToken ct)
     {
         var slot = _slots.GetOrAdd(mode, _ => new ModeSlot());
         lock (slot.Lock)
         {
             // 1. Already-resolved match cached for this viewer? Consume + return.
+            //    This caller is the FIRST arriver picking up their cached pair — owner role.
             if (slot.Resolved.TryGetValue(player.ViewerId, out var cached))
             {
                 slot.Resolved.Remove(player.ViewerId);
-                return Task.FromResult<PendingMatch?>(cached);
+                return Task.FromResult<PairUpResult?>(new PairUpResult(cached, IsOwner: true));
             }
 
             // 2. Someone already waiting in this slot? Pair with them.
+            //    This caller is the SECOND arriver who triggered the pair — joiner role.
             if (slot.Waiting is not null)
             {
                 if (slot.Waiting.ViewerId == player.ViewerId)
                 {
                     // Same viewer polled twice while parked — keep them parked.
-                    return Task.FromResult<PendingMatch?>(null);
+                    return Task.FromResult<PairUpResult?>(null);
                 }
                 var p1 = slot.Waiting;
                 var p2 = player;
@@ -44,12 +46,12 @@ public sealed class InProcessPairUp : IMatchingPairUpService
                 var match = _bridge.RegisterBattle(p1, p2, BattleType.Pvp);
                 // Cache the result for the FIRST arriver's next poll (consume-on-read).
                 slot.Resolved[p1.ViewerId] = match;
-                return Task.FromResult<PendingMatch?>(match);
+                return Task.FromResult<PairUpResult?>(new PairUpResult(match, IsOwner: false));
             }
 
             // 3. Empty slot — park this caller.
             slot.Waiting = player;
-            return Task.FromResult<PendingMatch?>(null);
+            return Task.FromResult<PairUpResult?>(null);
         }
     }