Step 3 of multi-instancing migration. The dominant per-battle singleton now
resolves through BattleAmbient.Current.Mgr when a scope is active. The legacy
'main' field is renamed _mainFallback and retained for unwrapped callers
(tests, anything not yet scope-wrapped). GetIns() still returns null when
neither is set, preserving the '?.Foo ?? default' patterns in engine code.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Step 2 of multi-instancing migration. Both flags now resolve through
BattleAmbient.Current when a scope is active, otherwise hit a static fallback
that preserves today's behavior unchanged for unwrapped callers.
Suite green: SVSim.BattleEngine.Tests pass; SVSim.UnitTests baseline holds.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
