PRE-GHR ET#6: The Numbered Fall — Causal Anchor Protocol: Minimal Non-Repudiable Sequences of Irreversible Forks (Empirical Trace #6)

How much of a system's causal history must be kept to keep the system honest? The Sender Axiom of this series answers: causal traces cannot be fully erased — there is a mathematical floor to what can be discarded (PRE-GHR XXXVIII, XXXIX). This trace explores the engineering counterpart: is there a storage ceiling — a way to keep far less than the full trace, while still binding the system to its irreversible decisions? The Causal Anchor Protocol (CAP) proposes exactly that: do not record all causal traces; record only anchor points at irreversible forks, chained so that each anchor commits to its predecessor. The paper is written as a self-demonstrating erratum: the protocol's original claim of tamper-impossibility (Material L57) is shown to be false for a purely introspective hash chain, and the correction — periodic anchoring of the chain head into heterogeneous external storage — is the load-bearing result. The honest version is stated in full: an introspective CAP can detect deletion but not rewriting; with external witnessing it detects both. The protocol's sparse-sampling criterion — anchor where the state cannot return — converges with the Sender Axiom at the class of send events, and only there. The anchor of a falling system is only as honest as the witness who holds its chain head. v1 (2026-08-16): content layer approved by red-pen review (2 must-fix: ORCID digit 0000→0009, out-of-material XIV references removed; 2 minor fixes: Q5.1 window dates, XL tense). Material: pre-ghr-et6-cap-material.md (md5 5c0d3eae). Material is the boundary; no fact beyond it is introduced. Series: PRE-GHR, Empirical Trace #6. License CC-BY-4.0.