V01.08 — Mirror Physics II: Constraint Salience and Physical Coupling

This paper is the second 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, which identified physical support conditions for recursive observer-continuity: ordered causal update, finite propagation, local substrate persistence, metastable memory, thermodynamic throughput and error-corrective repair. The present paper asks whether the organisational structure that makes such continuity possible can be assigned a disciplined mathematical quantity. It introduces constraint salience as a scale-relative and subsystem-relative functional over physical organisation. Constraint salience is not proposed as a new physical field, a modification of relativity or an additional force. It is a candidate bridge quantity intended to measure how strongly a local subsystem preserves viability-relevant structure through causal coupling, memory, error correction, recursive reliability and cost-sensitive repair. The paper specifies minimal requirements for such a quantity: operational measurability, explicit scale and horizon, compatibility with lower-level symmetries, thermodynamic accounting, component decomposability and recovery of established physics. It proposes a general salience functional, argues that ungoverned complexity is not sufficient for salience, distinguishes weak, intermediate and strong senses of physical coupling, and states the conditions that any future strong-coupling proposal would have to satisfy. The conclusion is conservative: Mirror Theory has not yet produced new physics, but it has isolated the mathematical form of the object that any future Mirror-physical extension would have to define, measure and constrain.