Add governed Statbel edition probe

docs/API_CONTRACTS.md

@@ -415,10 +415,15 @@ not marked stale merely because their source date is old. Unknown sources are
 ### GET `/api/v1/projects/{project_id}/datasets/source-catalog-probes`
 
 Performs an explicit, read-only release probe for the allowlisted GRB WFS,
-most-recent orthophoto WMS and ALZ agricultural-use parcel publication page.
+most-recent orthophoto WMS, Statbel population DCAT catalog and ALZ
+agricultural-use parcel publication page.
 For OGC services, the endpoint reads bounded `GetCapabilities` responses,
 confirms the expected layers and follows only HTTPS ISO 19139 `GetRecordById`
-links on `metadata.vlaanderen.be`. For ALZ, it reads only the bounded official
+links on `metadata.vlaanderen.be`. For Statbel, it reads the bounded official
+RDF/Turtle catalog and selects the latest uniquely identified Dutch
+`Bevolking per statistische sector` release. It requires the official landing
+page, CC BY 4.0 license and allowlisted distribution identities but never
+requests a ZIP or XLSX file. For ALZ, it reads only the bounded official
 HTML release page and accepts only exact
 `www.landbouwvlaanderen.be/bestanden/gis/agpa_<year>_<date>_public.zip` link
 identities. It does not request those archives. Query parameter `refresh=true`
@@ -438,6 +443,13 @@ edition as outdated. `service_type=HTML` uses `definitive_archive` and
 `current_snapshot` as evidence markers in the existing expected/matched/missing
 arrays; no parallel response shape is introduced.
 
+Statbel comparisons use the four-digit population reference year. The 2025
+release requires the new REDEGEO TXT/ZIP variant; its old-sector-layout file is
+reported only as transition evidence. `service_type=DCAT` uses
+`population_txt_current`, `landing_page` and `cc_by_4_0` as evidence markers.
+A newer statistical-sector geometry edition is not interpreted as a newer
+population release.
+
 The endpoint accepts no arbitrary URL, feature query, area or layer. It does
 not fetch vector features, raster pixels or models, create jobs/datasets, write
 to PostGIS or trigger an import. The normal `source-freshness` endpoint remains