feat(emulated-entrypoint): InProcessPairUp service for TK2 PvP matching

Tiny per-mode FCFS slot. First poller parks; second pairs and triggers
bridge.RegisterBattle(p1, p2, Pvp). Match cached for first poller's
next poll (consume-on-read). No MMR, no cross-mode, no timeouts --
the proper queue API is a separate spec; this is the smallest thing
that lets TK2 PvP work end-to-end.

SVSim.EmulatedEntrypoint/Matching/IMatchingPairUpService.cs

@@ -0,0 +1,20 @@
+using SVSim.BattleNode.Bridge;
+
+namespace SVSim.EmulatedEntrypoint.Matching;
+
+/// <summary>
+/// Minimal in-process matching queue stand-in. The proper queue API is a separate
+/// spec; this is enough to actually pair two viewers polling /do_matching on the
+/// same mode.
+/// </summary>
+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
+    /// (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).
+    /// </summary>
+    Task<PendingMatch?> TryPairAsync(string mode, BattlePlayer player, CancellationToken ct);
+}

SVSim.EmulatedEntrypoint/Matching/InProcessPairUp.cs

@@ -0,0 +1,62 @@
+using System.Collections.Concurrent;
+using SVSim.BattleNode.Bridge;
+using SVSim.BattleNode.Sessions;
+
+namespace SVSim.EmulatedEntrypoint.Matching;
+
+/// <summary>
+/// In-process FCFS pair-up: one waiting slot per mode, plus a per-viewer cache of
+/// resolved matches for the first arriver's next poll (consume-on-read).
+/// </summary>
+public sealed class InProcessPairUp : IMatchingPairUpService
+{
+    private readonly IMatchingBridge _bridge;
+    private readonly ConcurrentDictionary<string, ModeSlot> _slots = new();
+
+    public InProcessPairUp(IMatchingBridge bridge)
+    {
+        _bridge = bridge;
+    }
+
+    public Task<PendingMatch?> 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.
+            if (slot.Resolved.TryGetValue(player.ViewerId, out var cached))
+            {
+                slot.Resolved.Remove(player.ViewerId);
+                return Task.FromResult<PendingMatch?>(cached);
+            }
+
+            // 2. Someone already waiting in this slot? Pair with them.
+            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);
+                }
+                var p1 = slot.Waiting;
+                var p2 = player;
+                slot.Waiting = null;
+                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);
+            }
+
+            // 3. Empty slot — park this caller.
+            slot.Waiting = player;
+            return Task.FromResult<PendingMatch?>(null);
+        }
+    }
+
+    private sealed class ModeSlot
+    {
+        public BattlePlayer? Waiting { get; set; }
+        public Dictionary<long, PendingMatch> Resolved { get; } = new();
+        public object Lock { get; } = new();
+    }
+}