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);
+}