feat(pack): /pack/exchange_gacha_point endpoint

Wires IGachaPointService.TryExchangeAsync into a controller endpoint.
Loads the viewer with the full cosmetic-grant graph (Cards, Sleeves,
Emblems, Degrees, LeaderSkins, MyPageBackgrounds) plus the gacha-point
balance + received marker, with AsSplitQuery to avoid the cartesian
explosion documented in project_ef_split_query. Returns BadRequest with
the outcome's error code on validation failure, or the post-state
reward_list on success.

Two integration tests: happy-path verifies card grant + balance debit +
received-marker persistence + post-state reward_list shape; insufficient-
balance path returns 400.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/PackController.cs

@@ -163,6 +163,37 @@ public class PackController : SVSimController
         };
     }
 
+    [HttpPost("exchange_gacha_point")]
+    public async Task<ActionResult<ExchangeGachaPointResponse>> ExchangeGachaPoint(
+        ExchangeGachaPointRequest request)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        // Load the viewer with the collections the service mutates (balances, received marker,
+        // cards, cosmetics). AsSplitQuery per project_ef_split_query memory.
+        var viewer = await _db.Viewers
+            .Include(v => v.GachaPointBalances)
+            .Include(v => v.GachaPointReceived)
+            .Include(v => v.Cards)
+            .Include(v => v.Sleeves)
+            .Include(v => v.Emblems)
+            .Include(v => v.Degrees)
+            .Include(v => v.LeaderSkins)
+            .Include(v => v.MyPageBackgrounds)
+            .AsSplitQuery()
+            .FirstAsync(v => v.Id == viewerId);
+
+        var outcome = await _gachaPoint.TryExchangeAsync(viewer, request.ParentGachaId, request.CardId);
+        if (!outcome.Success) return BadRequest(new { error = outcome.Error });
+
+        await _db.SaveChangesAsync();
+
+        return new ExchangeGachaPointResponse
+        {
+            RewardList = outcome.RewardList.ToList(),
+        };
+    }
+
     [HttpPost("open")]
     [HttpPost("/tutorial/pack_open")]
     public async Task<ActionResult<PackOpenResponse>> Open(PackOpenRequest request)