feat: carve Jajce river into terrain

docs/PROJECT_CONTEXT.md

@@ -341,8 +341,9 @@ but retain far sources whenever their authoritative metadata can still win.
 
 - a centered 512 m Terrain3D landscape split across four regions;
 - deterministic shaped terrain data and six game-owned surface layers;
-- a larger summit citadel, eight cascading houses, mill, full-span bridge,
-  rock-framed waterfall, mist, smoke, and a Terrain3D-sampled river ribbon;
+- a larger summit citadel, eight cascading houses, mill, full-span bridge, and
+  a deterministic Terrain3D watercourse with an upper stream, raised waterfall
+  shelf, rock-framed drop, plunge basin, mist, and curved downstream ribbon;
 - authored dirt path strips that make the current village task loop readable;
 - compact silhouette props for the ridge landmark, guard, study, and rest
   sites, plus a physical pantry whose empty/low/stocked arrangement follows
@@ -979,10 +980,12 @@ Raw samples and workload exclusions live in
 [Simulation scaling baseline 02](benchmarks/SIMULATION_SCALING_BASELINE_02.md)
 and [Loaded Resource Discovery 01](benchmarks/LOADED_RESOURCE_DISCOVERY_01.md).
 
-The next slice should return to the authored Jajce stage: sculpt the real
-Terrain3D river channel and waterfall shelf, rebake/validate navigation, and
-use the following foliage expansion to prove a bounded stable-ID resource
-anchor placement workflow beside decorative Terrain3D instances. Do not yet
+The authored Jajce watercourse and its Terrain3D-derived navigation rebake are
+complete. The next slice should prove a bounded stable-ID resource-anchor
+placement workflow beside decorative Terrain3D/intentional foliage instances
+at one riverbank or forest edge. It should validate nearby and farther loaded
+resource discovery, reachable interaction points, and a measured presentation
+budget without making decorative instances authoritative. Do not yet
 generalize the resource grid or introduce active/abstract simulation LOD.
 
 The remaining simulation-garden target still aims for: