One opt-in diagnostic sink for all strop crates. Producers never perform file
I/O or wait for the writer; an incomplete trace is always reported as such.
Capture is bounded (total bytes, total events, per-record bytes) and every
finished file ends with an explicit terminal TraceEnd marker, so a capped
or failed capture can never be mistaken for a complete one. Forensic values
over the per-record cap travel as ordered replay_chunk runs (schema 3)
that the reader reassembles strictly — or refuses loudly.