feat: complete Wallonia land cover and terrain sources

docs/DATA_SOURCES.md

@@ -792,12 +792,20 @@ metric semantics.
 
 ## Wallonia WALOUS land cover
 
-The official SPW `WAL_OCS_IA__2020` and `WAL_OCS_IA__2023` GeoTIFF archives
-provide comparable Walloon land-cover observations at native 1 m resolution in
-EPSG:3812. Runtime analysis reads only bounded windows from operator-
+The official SPW `WALOUS_OCS__2018`, `WAL_OCS_IA__2020` and
+`WAL_OCS_IA__2023` GeoTIFF archives provide a governed Walloon land-cover time
+series at native 1 m resolution in EPSG:3812. Runtime analysis reads only
+bounded windows from operator-
 provisioned, checksum-recorded source files and uses nearest-neighbour
 resampling for the governed 10 m analysis derivative.
 
+The 2018 source retains stacked two-digit codes. GeoIntel uses the official
+`Classe vue` mapping and records the full crosswalk with every derived Dataset;
+the 2018-only greenhouse class `62` is explicitly grouped with artificial
+constructions. The 2018 production method included manual consolidation and
+therefore remains a documented methodological break, not a silently identical
+annual observation.
+
 The 11 semantic classes use the non-contiguous source codes `1, 2, 3, 4, 5, 6,
 7, 8, 9, 80, 90`. In order these mean artificial ground, above-ground
 construction, railway, bare soil, surface water, rotating herbaceous cover,
@@ -805,12 +813,25 @@ continuous herbaceous cover, conifer trees above 3 m, deciduous trees above 3
 m, conifer woody cover up to 3 m and deciduous woody cover up to 3 m. Codes 80
 and 90 must never be normalized to invented classes 10 and 11.
 
-The official temporal extents are 2020-04-01 through 2020-04-24 and 2023-05-27
-through 2023-06-25. Metrics are estimated hectares from classified cells. They
+The official temporal extents are calendar year 2018, 2020-04-01 through
+2020-04-24 and 2023-05-27 through 2023-06-25. Metrics are estimated hectares
+from classified cells. They
 are not legal land use, ownership, individual tree counts, timber volume or
 water volume. The official catalogue reports overall accuracy per edition and
 also warns that accuracy varies by class and place.
 
+## Wallonia terrain elevation
+
+The official SPW `RELIEF_WALLONIE_MNT_1M_2021_2022` source is operator-
+provisioned once from the fixed Geoportail artifact. GeoIntel validates and
+checksums the full EPSG:3812, one-band, 1 m GeoTIFF, but reads and persists only
+bounded analysis windows. The default derivative is 5 m with bilinear
+resampling and exact Area masking. Heights remain metres in the Deuxieme
+Nivellement General vertical reference (EPSG:5710); they are never relabelled
+as TAW. SPW documents small interpolated gaps and about 0.12 m absolute
+altimetric accuracy. Terrain analysis returns height, relief and slope while
+keeping drainage, water depth and water volume unsupported.
+
 ## Bathymetry, inland profiles and maritime scope
 
 The official VHA Digital Atlas profile-point layer is the first operational