Step 7 of multi-instancing migration. Residual SVSim.UnitTests that touch
engine code directly are wrapped in TestBattleScope. EngineSessionGate is
deleted along with the _engineOwned bookkeeping in BattleSession; engine
setup is unconditional now that per-battle state is isolated on the ambient.
Gate-specific fallback branches in BattleSession.ShadowIngest are simplified.
Suite fully green (SVSim.UnitTests, SVSim.BattleEngine.Tests).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>