feat(replay): add BattleContextStore for start->finish handoff

Bridges the start-time -> finish-time gap. /finish carries neither
battle_id nor opponent identity; this store holds both for the finish
handler to compose a ViewerBattleHistory row.

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

SVSim.UnitTests/Services/InMemoryBattleContextStoreTests.cs

@@ -0,0 +1,58 @@
+using SVSim.Database.Services.Replay;
+
+namespace SVSim.UnitTests.Services;
+
+public class InMemoryBattleContextStoreTests
+{
+    private static BattleContext MakeCtx(long battleId = 1, int oppViewer = 0) => new(
+        BattleId: battleId,
+        BattleType: 2,
+        DeckFormat: 0,
+        TwoPickType: 0,
+        SelfClassId: 1,
+        SelfSubClassId: 0,
+        SelfCharaId: 1,
+        SelfRotationId: "0",
+        OpponentViewerId: oppViewer,
+        OpponentName: "Opponent",
+        OpponentClassId: 2,
+        OpponentSubClassId: 0,
+        OpponentCharaId: 1,
+        OpponentCountryCode: "",
+        OpponentEmblemId: 0,
+        OpponentDegreeId: 0,
+        OpponentRotationId: "0",
+        BattleStartTime: DateTime.UtcNow);
+
+    [Test]
+    public void Set_then_TakeFor_returns_ctx_and_clears()
+    {
+        var store = new InMemoryBattleContextStore();
+        var ctx = MakeCtx(battleId: 42);
+
+        store.Set(viewerId: 100, ctx);
+        var taken = store.TakeFor(100);
+
+        Assert.That(taken, Is.Not.Null);
+        Assert.That(taken!.BattleId, Is.EqualTo(42));
+        Assert.That(store.TakeFor(100), Is.Null, "second take must return null");
+    }
+
+    [Test]
+    public void TakeFor_missing_viewer_returns_null()
+    {
+        var store = new InMemoryBattleContextStore();
+        Assert.That(store.TakeFor(999), Is.Null);
+    }
+
+    [Test]
+    public void Set_overwrites_prior_context_for_same_viewer()
+    {
+        var store = new InMemoryBattleContextStore();
+        store.Set(viewerId: 100, MakeCtx(battleId: 1));
+        store.Set(viewerId: 100, MakeCtx(battleId: 2));
+
+        var taken = store.TakeFor(100);
+        Assert.That(taken!.BattleId, Is.EqualTo(2));
+    }
+}