Geometric Reaction: Reactive Gravitation — A Source-Typed Theory of Gravitation with General Relativity as a Recovered Einsteinian Metric Corollary

This paper develops Reactive Gravitation within the First Law of Geometric Reaction as a source-typed theory of gravitation. It distinguishes reactive sourcing from baseline container structure and shows how General Relativity is recovered as an Einsteinian metric corollary on domains where all sourced stress-energy contributions are reactive. The paper also states domain-scoped observational handles and falsifiers for the reactive-only sourcing posture.