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.

scripts/assemble_belgium_building_corpus.py

@@ -102,6 +102,7 @@ def main() -> int:
     parser.add_argument("--output-dir", type=Path, required=True)
     parser.add_argument("--version", default="building-be-v1")
     parser.add_argument("--min-label-px", type=float, default=3.0)
+    parser.add_argument("--merge-touching-roofs", action="store_true")
     parser.add_argument("--freeze", action="store_true")
     args = parser.parse_args()
 
@@ -150,7 +151,7 @@ def main() -> int:
                 ),
                 reference_observed_at=reference.observed_at.isoformat() if reference.observed_at else None,
                 imagery_valid_to=raster.valid_to.isoformat() if raster.valid_to else None,
-                merge_touching_roofs=True,
+                merge_touching_roofs=args.merge_touching_roofs,
             )
             normalized_target.write_text(json.dumps(normalized, ensure_ascii=False), encoding="utf-8")
             audit_target.write_text(json.dumps(audit, ensure_ascii=False, indent=2), encoding="utf-8")