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.Database/Services/Replay/InMemoryBattleContextStore.cs

@@ -0,0 +1,20 @@
+using System.Collections.Concurrent;
+
+namespace SVSim.Database.Services.Replay;
+
+/// <summary>
+/// <see cref="ConcurrentDictionary{TKey, TValue}"/>-backed in-memory store.
+/// Lives as a singleton in DI. Server restart drops in-flight contexts —
+/// acceptable per spec (history is best-effort; finish handlers warn-log
+/// and continue when context is missing).
+/// </summary>
+public sealed class InMemoryBattleContextStore : IBattleContextStore
+{
+    private readonly ConcurrentDictionary<long, BattleContext> _contexts = new();
+
+    public void Set(long viewerId, BattleContext ctx)
+        => _contexts[viewerId] = ctx;
+
+    public BattleContext? TakeFor(long viewerId)
+        => _contexts.TryRemove(viewerId, out var ctx) ? ctx : null;
+}