feat(card-master): add ICardMasterPayloadProvider singleton, load blob at startup

SVSim.EmulatedEntrypoint/Services/CardMasterPayloadProvider.cs

@@ -0,0 +1,42 @@
+using Microsoft.Extensions.Logging;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Reads the static card-master blob from the app's output directory once at startup and
+/// hands the cached base64 string to <c>ImmutableDataController</c>. Singleton because the
+/// file is ~1.27 MB and gzip-decoding it on every request would burn CPU for no benefit.
+/// </summary>
+public interface ICardMasterPayloadProvider
+{
+    /// <summary>True when the blob loaded successfully and serving is enabled.</summary>
+    bool IsAvailable { get; }
+
+    /// <summary>The verbatim base64 string ready for <c>data.card_master</c>.</summary>
+    string Base64Blob { get; }
+}
+
+public sealed class CardMasterPayloadProvider : ICardMasterPayloadProvider
+{
+    // Filename is pinned to the captured snapshot's date. When swapping the blob, update
+    // this constant AND the hash in CardMasterConfig in the same change.
+    private const string BlobFileName = "card_master_2026-06-03.txt";
+
+    public bool IsAvailable { get; }
+    public string Base64Blob { get; }
+
+    public CardMasterPayloadProvider(ILogger<CardMasterPayloadProvider> log)
+    {
+        var path = Path.Combine(AppContext.BaseDirectory, "Data", BlobFileName);
+        if (!File.Exists(path))
+        {
+            log.LogWarning("Card-master blob missing at {Path} — /immutable_data/card_master will 503.", path);
+            Base64Blob = "";
+            IsAvailable = false;
+            return;
+        }
+        Base64Blob = File.ReadAllText(path).Trim();
+        IsAvailable = Base64Blob.Length > 0;
+        log.LogInformation("Loaded card-master blob ({Bytes} base64 chars).", Base64Blob.Length);
+    }
+}