feat(arena-tk2): PvP pair-up trigger via /do_matching, ?scripted=1 opt-in

Solo pollers park (3001 RETRY); two concurrent pollers pair and both
receive 3004 + same BattleId. Cache hits on the first arriver's next
poll. ?scripted=1 retains today's solo Scripted path for dev work.
Response DTO's BattleId/NodeServerUrl become nullable so 3001 omits
them on the wire (WhenWritingNull policy drops them).

ASP.NET's default bool binder rejects "1" as a value, so the scripted
opt-in is bound as string? and parsed permissively (accepts "1" and
"true"/"True"/etc.) rather than relying on built-in bool binding.

SVSim.EmulatedEntrypoint/Controllers/ArenaTwoPickBattleController.cs

@@ -1,5 +1,6 @@
 using Microsoft.AspNetCore.Mvc;
 using SVSim.BattleNode.Bridge;
+using SVSim.EmulatedEntrypoint.Matching;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Requests.ArenaTwoPick;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Responses.ArenaTwoPick;
 using SVSim.EmulatedEntrypoint.Services;
@@ -12,33 +13,67 @@ public class ArenaTwoPickBattleController : SVSimController
     private readonly IArenaTwoPickService _svc;
     private readonly IMatchingBridge _matching;
     private readonly IMatchContextBuilder _matchContextBuilder;
+    private readonly IMatchingPairUpService _pairUp;
 
     public ArenaTwoPickBattleController(
         IArenaTwoPickService svc,
         IMatchingBridge matching,
-        IMatchContextBuilder matchContextBuilder)
+        IMatchContextBuilder matchContextBuilder,
+        IMatchingPairUpService pairUp)
     {
         _svc = svc;
         _matching = matching;
         _matchContextBuilder = matchContextBuilder;
+        _pairUp = pairUp;
     }
 
     [HttpPost("do_matching")]
-    public async Task<IActionResult> DoMatching([FromBody] DoMatchingRequest req)
+    public async Task<IActionResult> DoMatching(
+        [FromBody] DoMatchingRequest req,
+        [FromQuery(Name = "scripted")] string? scripted = null,
+        CancellationToken ct = default)
     {
         if (!TryGetViewerId(out var vid)) return Unauthorized();
+        // Accept "1" or "true" (case-insensitive) as opt-in for the legacy Scripted path.
+        // ASP.NET's default bool binder rejects "1", so do a permissive parse here.
+        var useScripted = scripted is not null
+            && (scripted == "1" || string.Equals(scripted, "true", StringComparison.OrdinalIgnoreCase));
         try
         {
             var ctx = await _matchContextBuilder.BuildForTwoPickAsync(vid);
-            var match = _matching.RegisterBattle(
+
+            if (useScripted)
+            {
+                var scriptedMatch = _matching.RegisterBattle(
+                    new SVSim.BattleNode.Bridge.BattlePlayer(vid, ctx),
+                    p2: null,
+                    SVSim.BattleNode.Sessions.BattleType.Scripted);
+                return Ok(new DoMatchingResponseDto
+                {
+                    MatchingState = 3004,
+                    BattleId = scriptedMatch.BattleId,
+                    NodeServerUrl = scriptedMatch.NodeServerUrl,
+                });
+            }
+
+            var paired = await _pairUp.TryPairAsync(
+                "arena_two_pick_battle",
                 new SVSim.BattleNode.Bridge.BattlePlayer(vid, ctx),
-                p2: null,
-                SVSim.BattleNode.Sessions.BattleType.Scripted);
+                ct);
+            if (paired is null)
+            {
+                return Ok(new DoMatchingResponseDto
+                {
+                    MatchingState = 3001,
+                    // BattleId / NodeServerUrl null — client polls again.
+                });
+            }
+
             return Ok(new DoMatchingResponseDto
             {
                 MatchingState = 3004,
-                BattleId = match.BattleId,
-                NodeServerUrl = match.NodeServerUrl,
+                BattleId = paired.BattleId,
+                NodeServerUrl = paired.NodeServerUrl,
             });
         }
         catch (ArenaTwoPickException ex)

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaTwoPick/DoMatchingResponseDto.cs

@@ -17,10 +17,10 @@ public sealed class DoMatchingResponseDto
     public int RetryPeriod { get; set; } = 3;
 
     [JsonPropertyName("battle_id")] [Key("battle_id")]
-    public string BattleId { get; set; } = "";
+    public string? BattleId { get; set; }
 
     [JsonPropertyName("node_server_url")] [Key("node_server_url")]
-    public string NodeServerUrl { get; set; } = "";
+    public string? NodeServerUrl { get; set; }
 
     // Required by the client when matching_state ∈ {3004, 3007, 3011} —
     // DoMatchingBase.SettingCardMasterId does jsonData["card_master_id"].ToInt()