feat(battle-node): derive Matched.seed + Ready.idxChangeSeed from master seed

InitBattle now emits Stable(master) as the shared effect seed and the master-
shuffled deck as selfDeck; Swap emits each recipient's per-side IdxChange seed.
BattleSession exposes + logs the master seed per battle for future replay.
Updated lifecycle/dispatch/integration tests (deck assertions now permutation-
based since selfDeck is shuffled).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/Dispatch/Handlers/InitBattleHandler.cs

@@ -25,7 +25,8 @@ internal sealed class InitBattleHandler : IFrameHandler
             {
                 new(ctx.From, ServerBattleFrames.BuildMatched(
                     ctx.From.Context, ctx.Other.Context, ctx.From.ViewerId, ctx.Other.ViewerId,
-                    ctx.BattleId, BattleFrameDefaults.BattleSeed), false),
+                    ctx.BattleId, BattleSeeds.Stable(ctx.State.MasterSeed),
+                    ctx.State.GetShuffledDeck(ctx.From)), false),
             };
             ctx.SenderPhase = BattleSessionPhase.AwaitingLoaded;
             return r;