BU46 Existence-Observability Minimal Interface Axiom under B_U Partial Readout, Non-Invertible Observation, and Non-Surjective Model Space

Existence–Observability Minimal Interface Axiom under B_U separates existence from observability by defining them as two distinct domains. Existence belongs to the standing domain: the total-reality network locked by single-universe boundary consistency B_U and the fixed boundary B_0. Observability belongs to the readout domain: the settleable projection-stratum reality produced within a given resolution window. The paper introduces the minimal observation interface \mathcal{O}_{L_n}:\mathsf{Ex}_{B_0}\dashrightarrow \mathsf{Obs}_{L_n}, a partial, non-invertible, and non-surjective readout operator. Partiality means not every standing state enters the current observation window. Non-invertibility means a given observation cannot recover the full upstream state, projection path, or higher-order retained chain. Non-surjectivity means not every object writable in observational language, instrument format, or model parameter space corresponds to an actual standing state. This interface cuts three common errors: treating the unobservable as nonexistent, treating one readout as full source recovery, and treating formal describability as real standing. The paper further defines the thing-in-itself / phenomenon boundary as an audit line: phenomenon begins when a projection-level retention chain becomes readable, coherent, auditable, and settleable. Observability is therefore not existence itself, but a boundary-admitted readout of existence.