Refresh Belgian training gap evidence

docs/BELGIUM_BUILDING_TRAINING_LOOP.md

@@ -47,10 +47,14 @@ simultaneous gate.
 
 ## Current gap inventory
 
-The v4 dated corpus closes basic composition, leakage and temporal-identity
-gaps with 75 independent AOIs and 13,765 accepted building labels. It adds
-coastal, ribbon-development, farmland, park, forest and additional urban
-contexts. Its remaining known gaps are:
+The active v31 rotated-holdout corpus contains 142 independent AOIs and 26,041
+accepted source building labels. Its automated corpus audit has no failures:
+spatial leakage is zero, temporal identity is explicit and 272 sub-resolution
+plus 393 post-imagery labels are rejected rather than silently trained. The
+tile-quality audit records 51,244 visible training instances, zero invalid or
+missing label files, zero low-variance positive tiles and 214 negative tiles.
+It covers coastal, ribbon-development, farmland, park, forest, industrial,
+rail, quarry and additional urban contexts. Its remaining known gaps are:
 
 - dated 2025 imagery is used for Flanders and Brussels and the dated 2024 SPW
   campaign for Wallonia. Exact flight days remain a later metadata refinement,
@@ -58,8 +62,13 @@ contexts. Its remaining known gaps are:
 - GRB/PICC/UrbIS describe ground footprints, whereas visible roofs can remain
   displaced. The existing detector QA contract therefore uses IoU 0.25; the
   threshold is frozen and cannot be relaxed per candidate;
-- the first loop candidate generalises poorly in Flanders and Wallonia,
-  especially Mechelen, Sint-Niklaas, Leuven, Mons and dense PICC areas;
+- the active v38 loop still has to prove the frozen regional calibration gates;
+  iteration 2 improved aggregate calibration F1 to 0.573 but remained below
+  the Flemish F1/precision/recall gates and the Walloon precision gate;
+- Oostende coastal fabric is the dominant observed Flemish calibration
+  failure. Independent train-only coastal positives and port/dunes negatives
+  are therefore selected by the failure-driven sampler without opening or
+  copying protected calibration data;
 - sparse hard contexts pass the empty-image test more easily than dense urban
   recall, so both gates must remain independent;
 - building boxes are a valid first detector contract, but footprint-perfect
@@ -67,7 +76,7 @@ contexts. Its remaining known gaps are:
 
 The active production model remains unchanged while any gate fails.
 
-The v31 expansion adds eighteen independent train-only AOIs after the v30
+The v31 expansion added eighteen independent train-only AOIs after the v30
 rotated-holdout calibration isolated the remaining domain gaps. Nine Flemish
 AOIs cover industrial roofs, ribbon development and coastal urban fabric. Nine
 Walloon AOIs add dense urban, rural-town, regional-architecture and difficult