fix: judge robustness under the recommended exit, not the abandoned one

The robustness warning was computed on the target-model distribution
while the same panel recommends the hold exit — internally inconsistent.
_robustness_stats (median, profit factor, ex-top-5% expectancy) is now
shared by _bucket_stats and _time_exit_bucket, the time-exit table shows
Median Net R and Ex-Top-5% per hold length, and _build_recommendation
reads the trimmed expectancy from the recommended exit's bucket (falling
back to the target model when no hold is recommended).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/test_backtest_service.py

@@ -143,6 +143,10 @@ class TestTimeExitBucket:
         # No stop_day on any candidate → every hold runs the full 5 days.
         assert b["avg_hold_days"] == 5.0
         assert b["net_r_per_day"] == pytest.approx(0.28 / 5.0, abs=0.001)
+        # robustness on net rs [1.38, -1.02, 0.48]
+        assert b["median_net_r"] == pytest.approx(0.48, abs=0.001)
+        assert b["profit_factor"] == pytest.approx(1.86 / 1.02, abs=0.01)
+        assert b["net_avg_r_ex_top5"] == pytest.approx((0.48 - 1.02) / 2, abs=0.001)
 
     def test_missing_hold_skipped(self):
         b = bt._time_exit_bucket([{"time_r": {5: 1.0}}], 21)
@@ -341,7 +345,7 @@ def test_build_recommendation_reads_the_report():
         "overall_qualified": {"net_avg_r": 0.13, "net_avg_r_ex_top5": 0.05},
         "time_exit_sweep": [
             {"hold_days": 21, "net_avg_r": 0.38},
-            {"hold_days": 30, "net_avg_r": 0.50},
+            {"hold_days": 30, "net_avg_r": 0.50, "net_avg_r_ex_top5": 0.21},
         ],
         "gate_ablation": [
             {"variant": "all_floors", "total": 100, "hold_net_avg_r": 0.50},
@@ -374,7 +378,12 @@ def test_build_recommendation_reads_the_report():
     assert "keep the NEUTRAL exclusion" in gate_texts
     assert "80" in by_topic["cutoff"][0]
     assert "beats" in by_topic["benchmark"][0]
-    assert any("not a handful of outliers" in t for t in by_topic["robustness"])
+    # robustness is judged under the RECOMMENDED exit (the 30d hold), not the
+    # target model the recommendation advises abandoning
+    assert any(
+        "not a handful of outliers" in t and "under the recommended 30d hold" in t
+        for t in by_topic["robustness"]
+    )
 
 
 def test_build_recommendation_flags_outlier_dependence():