The Observer as a Record-Bearing Subsystem

This is the tenth paper of a series that redescribes physical reality as a causally consistent history of information updates, and the first panel of its second triptych - observer, then probability and typicality, then epistemic trust - ordered by dependence, since one cannot weigh observers before saying what counts as one. The ninth paper reasoned about observer typicality, Boltzmann brains, and a cosmological measure without ever saying, independently, what an observer physically is. This paper retires that undefined term. It declines the framing "for whom does a record exist" - in this framework records exist, and can even be public, with no one reading them - and asks instead for what kind of operationally individuated physical subsystem records can be acquired, integrated, internally decoded, and made causally relevant to later processing. The thin "any interacting system" notion of relational quantum mechanics is too thin for this epistemic and cosmological role; it does not by itself specify what physical structure carries records and licenses diachronic identification across interactions, and the paper develops a specific physical implementation layer, centred on endogenous accessibility and record-conditioned causal use, that the thin observer notion does not itself specify. The answer is two-layered. A candidate observer is individuated relative to a boundary, domain, timescale, tolerance vector, and endogenous operations-and-resources profile, under non-arbitrariness conditions permitting nested, overlapping, and distributed individuations - flagged as an explicit, unresolved observer-grain problem. Against a graded hierarchy of interactor, detector, recorder, observer, and agent, the paper separates an observer architecture, which implements four capacities, from an observation episode, which exercises them. Its sharpest commitment is endogenous accessibility, or self-legibility: a physically implemented internal decoding channel under the system's own resources - not a decoder in the theorist's description and not an inner reader. A companion two-relation account characterizes observer-lineage continuation across relational time, distinguishing architectural from epistemic continuation rather than asserting identity. The paper makes no independent empirical prediction; it derives neither quantum theory, the Born rule, nor a measure over observers, and offers no theory of consciousness.