feat(battlenode): SessionBattleEngine skeleton + types (Phase 2 N0)

SessionContentsCreator mirrors the test HeadlessContentsCreator fully (all
IBattleMgrContentsCreator members) so it compiles; Setup/Receive throw pending
the Task 3/4 probes. New files use the 'engine' extern alias.

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

SVSim.BattleNode/Sessions/Engine/EngineIngestResult.cs

@@ -0,0 +1,9 @@
+namespace SVSim.BattleNode.Sessions.Engine;
+
+/// <summary>Outcome of feeding one client frame to the engine (design ND6). A divergence/reject is a
+/// DETECTED-DESYNC EVENT surfaced to the caller — never silently absorbed. Phase-2 policy: log.</summary>
+internal sealed record EngineIngestResult(bool Accepted, bool Diverged, string? RejectReason)
+{
+    public static EngineIngestResult Ok() => new(Accepted: true, Diverged: false, RejectReason: null);
+    public static EngineIngestResult Reject(string reason) => new(Accepted: false, Diverged: true, RejectReason: reason);
+}