The Precision Cascade Engine is a domain-agnostic computational framework for resolving cross-domain scientific inconsistencies through explicit covariance modeling, systematic bias handling, and deterministic reproducibility enforcement. Papers 1–5 of the Precision Cascade series apply this engine to cosmology, neutrino physics, cosmic geometry, dark matter, and general relativity. This paper provides the complete engine-room specification underlying those results. It formally defines the universal architecture, canonical data structures, correlation and covariance construction rules, systematic bias engine, joint consistency and significance computation, precision cascade law, governance model, and reproducibility guarantees implemented across all published reproducibility archives. Paper 0 serves as the authoritative methods and governance document linking the primary scientific papers to their executable reproducibility packs. It explicitly defines the engine’s capabilities, non-capabilities, validation semantics, and failure modes, ensuring audit-grade transparency, determinism, and extensibility across scientific domains. Description What this record contains This record contains Paper 0 — The Engine Room, the authoritative specification of the Precision Cascade Engine. Paper 0 is not a results paper. It is the universal methods, architecture, and governance document that formally defines the computational engine used to generate all results in the Precision Cascade series. Role within the Precision Cascade series The Precision Cascade series consists of: Paper 1 — Resolution of the Hubble tension Paper 2 — Precision Cascade formalism and neutrino implications Paper 3 — Cosmic geometry and flatness validation Paper 4 — Detection of dark matter self-interactions (SIDM) Paper 5 — Large-scale validation of General Relativity Paper 6 — AI-assisted scientific discovery methodology Each of Papers 1–5 is accompanied by a self-auditing, deterministic reproducibility pack published separately. Paper 0 defines the engine that all those packs instantiate. What Paper 0 formally specifies Paper 0 defines, in full: A domain-agnostic modular architecture shared across all scientific applications Canonical data schemas for measurements, correlations, systematics, cascade factors, and golden outputs Explicit correlation and covariance construction rules, including cross-domain independence handling A transparent systematic bias engine with full provenance logging A joint consistency and significance computation framework using full covariance The Precision Cascade law, clarifying how stabilized parameters reduce uncertainty volume across domains Deterministic validation, golden-output, and FAIL→PASS semantics Governance rules covering versioning, provenance, reproducibility guarantees, and extensibility What this paper does not do To prevent misinterpretation, Paper 0 explicitly states that the engine: does not introduce new data does not refit raw likelihoods does not infer causal physics does not override published uncertainties does not amplify evidence or generate significance beyond declared covariance does not replace domain-specific scientific modeling The Precision Cascade Engine is a consistency and reproducibility framework, not a physics oracle. Relationship to reproducibility archives All published Precision Cascade reproducibility packs: implement the architecture defined in Paper 0 adhere to its deterministic execution and governance rules validate results against declared golden outputs are audited using published verification logic Paper 0 is therefore the single authoritative reference for how all Precision Cascade results are computed, validated, and audited. Intended audience This record is intended for: scientists evaluating the methodological foundations of the Precision Cascade results reproducibility and audit specialists institutions assessing determinism, transparency, and governance of computational research Citation guidance When citing results from Papers 1–5, this record should be cited as the methods and governance reference defining the Precision Cascade Engine used to produce those results.