feat(inventory): BeginAsync loads viewer with canonical graph

Includes Cards/Sleeves/Emblems/LeaderSkins/Degrees/MyPageBackgrounds/Items
under AsSplitQuery, plus caller-supplied extras via InventoryLoadConfig.
Opens a DB transaction and returns an InventoryTransaction shell. All
mutation methods throw NotImplementedException until subsequent tasks
land them.

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

SVSim.UnitTests/Services/Inventory/InventoryServiceBeginTests.cs

@@ -0,0 +1,50 @@
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database.Services.Inventory;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Services.Inventory;
+
+public class InventoryServiceBeginTests
+{
+    [Test]
+    public async Task BeginAsync_loads_viewer_with_canonical_graph()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var scope = factory.Services.CreateScope();
+        var inv = scope.ServiceProvider.GetRequiredService<IInventoryService>();
+
+        await using var tx = await inv.BeginAsync(viewerId);
+
+        Assert.That(tx.Viewer, Is.Not.Null);
+        Assert.That(tx.Viewer.Id, Is.EqualTo(viewerId));
+        Assert.That(tx.Viewer.Cards, Is.Not.Null, "Cards collection must be loaded");
+        Assert.That(tx.Viewer.Sleeves, Is.Not.Null, "Sleeves collection must be loaded");
+        Assert.That(tx.Viewer.Items, Is.Not.Null, "Items collection must be loaded");
+    }
+
+    [Test]
+    public async Task BeginAsync_throws_when_viewer_missing()
+    {
+        using var factory = new SVSimTestFactory();
+        using var scope = factory.Services.CreateScope();
+        var inv = scope.ServiceProvider.GetRequiredService<IInventoryService>();
+
+        Assert.ThrowsAsync<InventoryViewerNotFoundException>(
+            async () => { await inv.BeginAsync(viewerId: 9999); });
+    }
+
+    [Test]
+    public async Task BeginAsync_applies_extra_includes_via_configure()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var scope = factory.Services.CreateScope();
+        var inv = scope.ServiceProvider.GetRequiredService<IInventoryService>();
+
+        await using var tx = await inv.BeginAsync(viewerId, configure:
+            cfg => cfg.WithInclude(v => v.MissionData));
+
+        Assert.That(tx.Viewer.MissionData, Is.Not.Null);
+    }
+}