feat(load-index): emit data.card_master_hash on mismatch (tier-1 freshness gate)

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/IndexResponse.cs

@@ -42,6 +42,23 @@ public class IndexResponse
     [Key("deck_format")]
     public Format DeckFormat { get; set; } = Format.Rotation;
 
+    /// <summary>
+    /// Freshness trigger for the card-master refresh flow (Wizard/CardMaster.cs:18-30).
+    /// Nullable + global <c>WhenWritingNull</c> means absence on the wire when the request
+    /// already matches <c>CardMasterConfig.CurrentHash</c>. Presence (any value) tells the
+    /// client to call <c>POST /immutable_data/card_master</c> with this echoed back; the
+    /// client treats the string as opaque.
+    /// <para>
+    /// Lives on the inner <c>data</c> payload, NOT <c>data_headers</c> — verified by
+    /// <c>LoadDetail.cs:414</c> constructing <c>new CardMaster.UpdateInfo(jsonData)</c>
+    /// from the inner data, and the 2026-06-03 capture at
+    /// <c>data_dumps/captures/traffic_prod_allstars_freepack.ndjson</c>.
+    /// </para>
+    /// </summary>
+    [JsonPropertyName("card_master_hash")]
+    [Key("card_master_hash")]
+    public string? CardMasterHash { get; set; }
+
     #endregion
 
     #region Basic User Data