Bounded transient departures around the GR-exact branch in flux-fixed GR: an admissible extension class and falsifier interface (v1.0.0) On the GR-exact, flux-fixed branch of the gauged constant vacuum-mode (GCV) framework, local dynamics coincide with general relativity with a flux-fixed integration constant Λeff once Λeff is fixed by late-time geometry. This note defines a conservative bounded-transient extension class around that baseline. The extension class is designed to answer a narrow question: under what conditions can departures from GR+Λ be admitted without reintroducing a local running vacuum term? The answer is phrased as a requirement set in which any allowed departure must be: excitation-driven vacuum-subtracted (no new constant mode) bounded by a small UV-controlled parameter transient relative to Λeff The note packages this as an admissible extension class, a bounded-departure lemma, and a survey-facing falsifier interface. Scope and claim boundary This note does not claim evidence for departures in current data. It does not propose a unique UV completion. It does not provide a full dynamical model for the departure profile. Its purpose is to define the allowed extension class clearly and to state what would falsify it. Positioning within the GCV programme The published GCV default remains the GR-exact late-time branch. The already-published optional response sector remains growth-only on a fixed background. The present note defines a stronger but still bounded extension class in which transient geometry excursions are admitted without reintroducing a local constant mode. Files in this record PDF manuscript clean source/release bundle Project homepage: https://johansson.digital Project overview hub: https://github.com/gcv-framework/gcv-vacuum-energy