The Ontology of Vacuum: The N = 0 Ground State of the Constraint Network

This document establishes the constitutional ontology of vacuum within the Energy-Efficiency Theory (EET) framework. The vacuum is defined as the $N=0$ ground state of the constraint network---the state of the energy substrate with zero persistent Type I constraint nodes. It is not quiescent: deep transient constraint events (S₃ → S₂ → S₀) occur at an irreducible frequency governed by the minimum action quantum AEET = /2, producing a universal fluctuation spectrum SΔ(ω) = (/4π)|ω| (Vacuum Unquiescence Theorem, CLOSED-in-EET). The vacuum's transient dynamics are described by the weak Euler representation (eiθ without persistent modulus), the mathematical signature of non-Being---the substrate operating in its default, unconstrained mode. Version 2.1 is a v6.1 constitutional alignment, companion ontology synchronization, and constitutional deepening upgrade. It incorporates the Being/non-Being framework (Two Forms of Energy v3.4, 2--5), establishing the vacuum as the purest expression of Existence and the physical specification of non-Being. It registers the vacuum as the pre-bifurcation substrate---the physical state prior to the energy bifurcation into free-state and constrained-state forms. It establishes the vacuum as the five-channel zero baseline of the complete constraint Second Law (Constraint Network Dynamics v4.0, Part III-C.2)---the unique physical state in which all five entropy production channels have zero mean, with only the Transient channel possessing non-zero variance. It formalizes the vacuum as the constitutional null test of the EET framework (2.8): the single physical state in which every ``N ≥ 1'' constitutional structure undergoes systematic degeneration, confirming the integrated architecture of the EET constitutional system. It provides the vacuum's sevenfold ``pre-'' identity (pre-bifurcation, pre-constraint, pre-Being, pre-time, pre-space, pre-information, pre-interaction), identifies the vacuum as both the fixed point of the EET explanatory chain and the unique pure state without internal contradiction, and establishes the universal calibration baseline principle. It adds a comprehensive Appendix H, Providing detailed EET resolutions of sixteen major vacuum-related open problems in contemporary physics, with constitutional anchoring and priority registration. Appendix I documents the vacuum's philosophical resonances---from Hegel's dialectic of Being and Nothing to Aristotle's first matter, from the ancient void/plenum debate to Kant's limits of observation---as structural parallels grounded in the $N=0$ vacuum's physical constitution.The vacuum ontology maintains 29 interfaces to 24 companion ontologies, establishes the jurisdictional boundaries of all ``N ≥ 1'' constitutional concepts, and provides the physical foundation for EET's treatment of the cosmological constant problem, the ontological status of virtual particles, the ``something from nothing'' problem, and the origin of irreversibility. All v2.0 constitutional content is retained and expanded.{Keywords:} Vacuum ontology; $N=0$ ground state; constraint network; non-Being; pre-bifurcation substrate; transient constraint events; Vacuum Unquiescence Theorem; weak Euler representation; transient constraint template spectrum; five-channel zero baseline; constitutional null test; η definitional domain boundary; primordial formation (N:0→ 1); cosmological constant problem; virtual particles; Casimir effect; Being/non-Being framework; Existence; Difference Precedes Constraint; constitutional singularity; universal calibration baseline; vacuum--causality--CND constitutional triangle; three arrows of time.