When the reranker hung on rerank=True there were zero log lines after
'retrieved=N (no threshold)', which made it impossible to tell whether
the hang was in _load (CrossEncoder construction), in get_reranker's
lock acquisition, or in predict. Structlog routing may also be the
culprit since we never saw the 'Loading RAG reranker' info line.
Add unconditional stderr prints at each milestone — entered, device
resolved, before CrossEncoder, after CrossEncoder, rerank entered,
predict starting, predict done. These bypass any logger config and
show up directly in /tmp/studio.log next to the rest of the captured
stdout/stderr. Leaving structlog logger.info calls in place too so
the structured stream still gets the same data when routing works.
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