Update downloader and river erosion

PIPELINE.md

@@ -198,6 +198,24 @@ TREES-ENHANCED:
 
 ---
 
+### 7. River Erosion (Optional)
+```
+export_unity/height_png16/*.png ──► depth raster from HydroRIVERS (buffered by order)
+export_unity/tile_index.csv ───────► bounds + scaling
+                                  │
+                                  ▼
+Depth burn = MAX depth (emulated by sorting burns ascending + REPLACE)
+                                  │
+                                  ▼
+export_unity/height_png16_river/{tile}.png
+export_unity/tile_index_river_{vr|server}.csv
+```
+Notes:
+- Uses `raw/hydrorivers/.../HydroRIVERS_v10_eu.shp` and runs after heightmap export.
+- Rasterize merge supports only REPLACE/ADD; max-depth is achieved by ordered burns.
+
+---
+
 ## File Format Reference
 
 | Output | Format | Resolution | Purpose |