feat(gift): add GiftRewardTypes supported-set helper

Centralizes which UserGoodsType values IInventoryTransaction.GrantAsync
can handle in the gift-inbox flow; both GiftController (post-fix) and
CampaignController will consult this instead of duplicating the set.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Services/GiftRewardTypes.cs

@@ -0,0 +1,42 @@
+using SVSim.Database.Enums;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// The subset of <see cref="UserGoodsType"/> that <c>IInventoryTransaction.GrantAsync</c>
+/// can grant via the gift-inbox claim flow. Producers (serial codes, future event mailers)
+/// validate against this before creating <c>ViewerPresent</c> rows so unsupported types
+/// fail at production time rather than at the player's claim.
+/// </summary>
+/// <remarks>
+/// Excluded from gift grants:
+/// <list type="bullet">
+/// <item><c>SpotCard</c>, <c>SpotCardOnlyLatestCardPack</c> — InventoryTransaction throws NotSupportedException.</item>
+/// <item><c>FreeGachaCount</c> — InventoryTransaction has no grant case (default arm throws NotImplementedException).</item>
+/// </list>
+/// </remarks>
+public static class GiftRewardTypes
+{
+    public static bool IsSupported(UserGoodsType type) => type switch
+    {
+        UserGoodsType.Crystal       => true,
+        UserGoodsType.Rupy          => true,
+        UserGoodsType.RedEther      => true,
+        UserGoodsType.SpotCardPoint => true,
+        UserGoodsType.Item          => true,
+        UserGoodsType.Card          => true,
+        UserGoodsType.Sleeve        => true,
+        UserGoodsType.Emblem        => true,
+        UserGoodsType.Degree        => true,
+        UserGoodsType.Skin          => true,
+        UserGoodsType.MyPageBG      => true,
+        UserGoodsType.SpotCard                   => false,
+        UserGoodsType.SpotCardOnlyLatestCardPack => false,
+        UserGoodsType.FreeGachaCount             => false,
+        _ => false,
+    };
+
+    public static bool IsSupported(int wireType) =>
+        Enum.IsDefined(typeof(UserGoodsType), wireType) &&
+        IsSupported((UserGoodsType)wireType);
+}