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/BattleContext.cs

@@ -0,0 +1,26 @@
+namespace SVSim.Database.Services.Replay;
+
+/// <summary>
+/// Per-viewer battle context captured at start time (do_matching/start) and consumed
+/// at finish time. Lives in <see cref="IBattleContextStore"/> for the duration of a
+/// single battle. See docs/superpowers/specs/2026-06-10-replay-info-design.md.
+/// </summary>
+public sealed record BattleContext(
+    long   BattleId,
+    int    BattleType,
+    int    DeckFormat,
+    int    TwoPickType,
+    int    SelfClassId,
+    int    SelfSubClassId,
+    int    SelfCharaId,
+    string SelfRotationId,
+    int    OpponentViewerId,
+    string OpponentName,
+    int    OpponentClassId,
+    int    OpponentSubClassId,
+    int    OpponentCharaId,
+    string OpponentCountryCode,
+    long   OpponentEmblemId,
+    long   OpponentDegreeId,
+    string OpponentRotationId,
+    DateTime BattleStartTime);

SVSim.Database/Services/Replay/IBattleContextStore.cs

@@ -0,0 +1,17 @@
+namespace SVSim.Database.Services.Replay;
+
+/// <summary>
+/// In-memory per-viewer battle context store. Bridges the start-time → finish-time
+/// gap: the /finish request body carries neither battle_id nor opponent identity,
+/// so this stash holds everything the finish hook needs to compose a
+/// ViewerBattleHistory row.
+/// </summary>
+public interface IBattleContextStore
+{
+    /// <summary>Store the viewer's active battle context. Overwrites any prior entry.</summary>
+    void Set(long viewerId, BattleContext ctx);
+
+    /// <summary>Atomic read+clear. Returns null when no context (server restart,
+    /// non-tracked family, already taken). Finish handlers must tolerate null.</summary>
+    BattleContext? TakeFor(long viewerId);
+}

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;
+}