Harden Statbel population import compatibility

docs/STORAGE_ARCHITECTURE.md

@@ -101,6 +101,27 @@ Do not delete originals automatically. Derived outputs may be cleaned through ex
 
 ## Official temporal source artifacts
 
+Statbel population staging stores immutable official source ZIPs below the
+selected operator output root:
+
+```text
+<population-output>/
+  raw/{year}/OPENDATA_SECTOREN_{year}[_NEW].zip
+  raw/{year}/sh_statbel_statistical_sectors_31370_{year}0101.geojson.zip
+  {scope}_statbel_population_{year}.geojson
+  {scope}_statbel_population_{year}.preflight.json
+```
+
+The preflight manifest records official URLs, archive/member size and SHA-256,
+schema fingerprints, EPSG:31370 and situation-date evidence, bounded topology
+repairs, national/scope join accounting, unlocated `ZZZZ` totals, baseline
+change evidence and the final derived-snapshot checksum. The upload operator
+revalidates all retained files against that manifest. Raw ZIPs and manifests
+are provenance artifacts; queryable population geometry remains an ordinary
+Dataset/DatasetVersion plus PostGIS `vector_features` through the canonical
+persistence services. Temporary standalone preflight output does not create a
+database record and may be removed explicitly after operator review.
+
 Waterinfo raw station layers, timeseries responses and checksum manifests live
 under `storage/operator-data/waterinfo/<scope>/`. These are immutable source
 evidence; queryable annual Point snapshots are normal Dataset/vector_feature