Stabilize near-gate regional sampling

backend/tests/test_failure_driven_yolo_sampling.py

@@ -251,6 +251,33 @@ def test_region_cap_rotates_repeats_between_sampling_rounds() -> None:
     assert second_metadata["sampling_round"] == 2
 
 
+def test_precision_guard_band_keeps_near_gate_region_stabilized() -> None:
+    manifest = {"samples": [
+        {"sample_slug": "wa-positive", "split": "train", "region": "wallonia", "context": "rural-town"},
+        {"sample_slug": "wa-negative", "split": "train", "region": "wallonia", "context": "farmland-hard-negative"},
+    ]}
+    summary = {"tiles": [
+        {"sample_slug": "wa-positive", "split": "train", "label_count": 1, "image_path": "/tmp/wa-positive.png"},
+        {"sample_slug": "wa-negative", "split": "train", "label_count": 0, "image_path": "/tmp/wa-negative.png"},
+    ]}
+    assessment = {
+        "status": "continue_training_loop",
+        "gates": {"min_region_f1": .45, "min_region_precision": .5, "min_region_recall": .4,
+                  "max_pure_empty_false_positives": 0},
+        "calibration": {"regions": {
+            "wallonia": {"f1": .6, "precision": .52, "recall": .7},
+        }},
+    }
+
+    paths, metadata = MODULE.build_sampling(
+        summary=summary, manifest=manifest, assessment=assessment, max_region_share=1.0,
+    )
+
+    assert "wallonia" in metadata["weak_precision_regions"]
+    assert paths.count(str(Path("/tmp/wa-negative.png").resolve())) == 4
+    assert metadata["precision_guard_band"] == .03
+
+
 def test_coastal_precision_failure_targets_port_and_dunes_negatives() -> None:
     manifest = {"samples": [
         {"sample_slug": "coastal-train", "split": "train", "region": "flanders", "context": "coastal-urban"},