V01.10 — Mirror Physics IV: Causal Invariance and the Speed of Constraint Propagation

This paper is the fourth paper in the Physics Support Arc of the Mirror Programme, Volume I: Observerhood. It follows V01.07 — Mirror Physics I: Constraint Propagation and the Physical Conditions for Observerhood, V01.08 — Mirror Physics II: Constraint Salience and Physical Coupling, and V01.09 — Mirror Physics III: Thermodynamic Observerhood. Mirror Theory treats observerhood as a recursive, viability-constrained process: a local system maintains a world-model, a self-model, reliability variables, memory governance and identity-continuity under perturbation. Earlier papers in the sequence developed the mathematical language of constraint regimes, observer thresholds, identity-continuity, physical support conditions, constraint salience and thermodynamic maintenance. The present paper turns to causal structure. It argues that persistent observerhood requires not merely information processing, but ordered and bounded causal update. In relativistic physics, the invariant speed c defines the boundary of causal influence and the structure of light cones. Mirror Theory does not modify this structure. Instead, it interprets c as the physical update bound within which memory, prediction, correction, self-modelling and identity-continuity can remain locally coherent. The central result is a Causal Support Theorem: any local observer that persists through time requires a physical regime supporting stable causal order, finite update propagation, local separability, memory preservation and non-instantaneous correction loops. The paper explicitly does not derive the numerical value of c, does not propose faster-than-light travel and does not alter relativity. Its contribution is interpretive and structural: it identifies why an invariant causal speed is not merely a feature of signal propagation, but a condition under which recursive observer-continuity can be physically supported.