2026-07-23 directive: instrument candidates-proposed-per-served-turn on
clean, post-reset traffic. Adds an additive-optional turn_count_baseline
manifest field (engine_state) stamping the turn_count at the last clean
reset of the discovery-candidate ledger, a pure compute_discovery_yield
function (teaching/discovery_yield.py), and a read-only `core teaching
discovery-yield` CLI surface. Fails closed (returns None / exits 1) when
no baseline has been stamped rather than fabricating a denominator.
scripts/ops/stamp_discovery_yield_baseline_20260723.py stamps the T11
reset epoch into the live store, mirroring reset_candidate_corpus_t11's
atomic generation-dir pattern. Dry-run verified against a throwaway copy
of the live store's gen-21703; not yet executed against the real store
(see PR body).
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