correct overlapping checkpoint evaluation

docs/reviews/2026-08-09-ai-assisted-checkpoint-review.md

@@ -187,3 +187,32 @@ visible; it was not suppressed or relabelled as success.
 This min-4px version is the preferred experimental successor to the legacy
 min-3px corpus. It explicitly remains ineligible for governed training until
 real human review and all release-contract evidence exist.
+
+## Non-overlapping checkpoint re-evaluation
+
+The 36-tile checkpoint matrix was traced to a 512 px validation corpus with
+stride 256. It contained nine views each of Turnhout, Westerlo, Postel-bos and
+Arendonk-heide. Visual inspection then exposed that all Arendonk-heide views
+are blank/no-data imagery, not meaningful pure-background observations. The
+historical claim of 18 background images is therefore corrected: nine were
+blank no-data and nine represented Postel under overlap.
+
+A training-disabled validation view now covers each 1024 px AOI with four
+non-overlapping 512 px tiles. Four blank Arendonk representatives are excluded
+with explicit reason codes. The resulting set contains eight positive tiles
+from Turnhout/Westerlo, four real Postel background tiles and 2,551 labels.
+Exact reconstruction finds 2,384 unique interior objects, 167 edge labels and
+zero repeated interior objects. The view has empty train directories, a
+checksum-bound `NO_TRAINING.json`, and the training wrapper rejects that marker.
+
+On the Tower RTX 4080 SUPER, the active checkpoint measures precision
+`0.542632`, recall `0.435317`, mAP50 `0.342034` and mAP50-95 `0.141316`.
+The reviewedexp6 challenger measures `0.581715`, `0.462300`, `0.368364` and
+`0.155318` respectively. At background confidence 0.15 the active checkpoint
+has zero Postel detections and the challenger has one. At 0.25 both have zero.
+Thus the refined evidence confirms the challenger's relative metric advantage
+and its threshold sensitivity, but still does not authorize promotion.
+
+Non-overlap is not overstated as statistical independence: the positive tiles
+remain adjacent and come from only two AOIs, edge objects can remain split, and
+there is only one real background AOI. The production model remains unchanged.