Merge: per-observation outlier rejection (env-gated PR_REJECT)

At the jet's ~1000x multiplicity R-free is bias-limited, and the merge had NO
outlier rejection - serial data zingers/overlaps/mis-indexed frames bias every
merged mean. Add a robust per-observation cut: drop observations whose corrected
intensity lies > reject_nsigma error-model sigmas from the reflection's MEDIAN.
The error-model sigma already captures the genuine (partiality) scatter, and the
median is a robust centre, so only the tail beyond the real spread is removed -
not good partials. The median is computed in RefineErrorModel (which already pools
the observations per reflection); AddImage applies the cut.

Env-gated via PR_REJECT=<nsigma> (off by default); logs the count removed. On the
jet (CC proxy) it lifts CCref +8 (nsigma 6, 0.6% cut) to +11 (nsigma 3, 7.4% cut)
- the cut is vs the data's own median, not the reference, so the gain is real
cleaner means. R-free validation + the nsigma sweet spot (over-rejection risk at
low nsigma) are for Filip's full-jet R-free.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/jfjoch_process.cpp

@@ -1034,8 +1034,15 @@ int main(int argc, char **argv) {
             const double isa = (b > 0.0) ? 1.0 / b : std::numeric_limits<double>::infinity();
             logger.Info("Error model: sigma'^2 = {:.3f} sigma^2 + ({:.4f} I)^2   ISa = {:.1f}", a, b, isa);
         }
+        if (const char *rj = std::getenv("PR_REJECT")) { // TEMP A/B: per-observation outlier rejection
+            const double nsig = std::atof(rj);
+            merge_engine.SetRejectOutliers(nsig);
+            logger.Info("Outlier rejection enabled: dropping observations > {:.1f} sigma from the per-reflection median", nsig);
+        }
         for (auto &i : indexer.GetIntegrationOutcome())
             merge_engine.AddImage(i);
+        if (merge_engine.RejectedCount() > 0)
+            logger.Info("Outlier rejection removed {} observations", merge_engine.RejectedCount());
 
         auto merged_reflections = merge_engine.ExportReflections();
         auto merged_statistics = merge_engine.MergeStats(merged_reflections, indexer.GetIntegrationOutcome(), reference_data);