Keep visible roof merging opt-in

docs/BELGIUM_BUILDING_TRAINING_LOOP.md

@@ -95,10 +95,10 @@ audit but excluded from training as `created_after_imagery_period`. UrbIS does
 not expose an equivalent feature creation field in this acquisition contract,
 so its remaining temporal relation stays an explicit sample-level limitation.
 
-The detector target is an imagery-visible roof instance, not an administrative
-address or cadastral unit. Source footprints that truly touch or overlap are
-dissolved into one roof instance before tiling; separated footprints are never
-bridged. The audit retains every contributing native feature identifier and
-reports both the accepted source-feature count and resulting visible-instance
-count. This prevents dense row-house subdivisions that cannot be distinguished
-from the orthophoto from becoming contradictory image labels.
+An opt-in visible-roof experiment can dissolve source footprints that truly
+touch or overlap; separated footprints are never bridged. The audit retains
+every contributing native feature identifier and reports both source-feature
+and visible-instance counts. This mode is not the default: the Belgium v8
+experiment showed that unconditional touching-footprint dissolve can merge
+whole urban blocks and therefore must pass the same independent gates before
+it can replace native instances.