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reproduce_family returns Result<ReproduceReport, RunnerError> — divergence is a result the embedding caller reads (all_identical()), never an error; the CLI renders the per-member lines and summary byte-identically and owns the exit (1 on divergence, no stderr line). The library's print block and the empty-message refusal are gone; exit_on_runner_error always prints (no empty producer remains). reproduce_family_in gains the identity/pip guard, firing before any store fetch: an instrument mismatch (incl. synthetic-for-real) and a broker/pip mismatch — forward-built label compare against the stored broker string (RunManifest carries no pip field; the mint's label constructors became shared pub(crate) helpers, so guard and stamp cannot drift). The class is context-borne: the explicit-source seam refuses 2 (caller-named source), the derived simple path refuses 1 (data drift), same prose. A window guard is deliberately absent — no reproduce path takes a caller window. In-process tests pin the promise: divergence returns Ok, drift refuses class 1, the host survives. Fork minutes: issues/299#issuecomment-4875, -4877, -4879. closes #299