Time as a Constrained Substrate: Admissibility, Relativity, and the Schwarzschild Lesson

Boundary and Scope Notice (CRL-0): This document is observer-only and non-authoritative. Any method-like descriptions, contrasts, figures, or illustrations are interpretive and non-normative; they are not offered as deployment guidance, certification criteria, or operational decision rules. No recommended calibration, thresholds, protocols, or implementation guidance are provided. Any evaluable instantiation belongs to downstream evaluation artifacts. Structural claims and results are unchanged. This paper introduces an interpretive substrate for the Constraint Geometry Series (Papers 1–5), framing time as a constrained, observer-relative substrate rather than as a global index or control variable. The paper does not propose a new physical theory, modify relativity, or introduce new dynamical laws. Instead, it clarifies why admissibility-based analyses naturally give rise to bounded dynamics, non-return regions, and irreversibility without invoking intent, optimization, or global clocks. Drawing on the methodological precedent of exact solutions in relativity, the paper emphasizes that invariant boundaries are revealed by excluding inadmissible structures rather than by prescribing behavior. Within this posture, irreversibility is presented geometrically as exclusion from admissible state space, not as a narrative about past and future. This note serves as an interpretive lens for the series, not a prerequisite for its results. It absorbs the preceding papers without explaining or justifying them and closes the compression phase of the Constraint Program while preparing the ground for subsequent reconstruction and application. The paper is non-operational by design and introduces no procedures, mechanisms, or implementation guidance.