APR-Agentic Phase 3 | Drift Evaluator v3 Specification: Empirical Basis and Design Rationale

This technical note documents the empirical basis and design rationale for the APR-Agentic Drift Evaluator v3, derived from a 194-event governance corpus generated by the APR-Lite substrate during Phase 3 harness testing. The corpus spans healthcare, legal, and SMB sectors under multi-topology stress conditions. Analysis reveals three principal findings: (1) privilege and root_intent engine co-occurrence constitutes the dominant cascade precursor pattern; (2) identity drift fires independently and requires a separate accumulation path; and (3) the substrate's drift engine detects pattern-level violations that single-engine evaluation misses, providing the strongest justification for a persistence-based evaluator. Semantic drift is deferred pending corpus expansion. This note establishes the conformance conditions the v3 evaluator must satisfy and is published prior to implementation to establish design priority and provide a stable reference for audit and regulatory review. Related work: This specification extends the APR-Agentic Phase 3 Drift Policy Architecture specification (DOI: 10.5281/zenodo.19154866) and the APR-Coordination Specification (DOI: 10.5281/zenodo.19139612). The evaluator design is grounded in the substrate-layer invariants defined in The Substrate: A Canonical Definition (DOI: 10.5281/zenodo.19133843).