feat(card-master): implement POST /immutable_data/card_master serving captured blob

SVSim.EmulatedEntrypoint/Controllers/ImmutableDataController.cs

@@ -0,0 +1,36 @@
+using Microsoft.AspNetCore.Mvc;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests.ImmutableData;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Responses.ImmutableData;
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.EmulatedEntrypoint.Controllers;
+
+/// <summary>
+/// Family <c>/immutable_data/*</c>. v1 hosts the single endpoint <c>card_master</c>;
+/// future siblings (e.g. mission masters, asset bundle hash tables) would land here.
+/// </summary>
+[Route("immutable_data")]
+public class ImmutableDataController : SVSimController
+{
+    private readonly ICardMasterPayloadProvider _provider;
+
+    public ImmutableDataController(ICardMasterPayloadProvider provider)
+    {
+        _provider = provider;
+    }
+
+    /// <summary>
+    /// Returns the base64+gzip+json+csv card-master payload. Tier 1 serves a static prod
+    /// snapshot regardless of the request's <c>card_master_hash</c>; freshness gating is
+    /// handled on <c>/load/index</c> instead.
+    /// </summary>
+    [HttpPost("card_master")]
+    public ActionResult<CardMasterResponse> CardMaster([FromBody] CardMasterRequest req)
+    {
+        if (!_provider.IsAvailable)
+        {
+            return StatusCode(503, "card-master payload not available");
+        }
+        return Ok(new CardMasterResponse { CardMaster = _provider.Base64Blob });
+    }
+}