Step 4 of multi-instancing migration. Three additional per-battle statics
front-fronted by BattleAmbient.Current, each with a static fallback for
unwrapped callers. ViewerId's SavedataManager-persisting setter is preserved
on the fallback path; inside a scope, the setter is a no-op (the per-battle
perspective is fixed at scope entry).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
