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);