Fix structures tearing apart and floating due to inconsistent baseY calculations

internal/worldgen/structure_manager.go

@@ -14,19 +14,37 @@ func PlaceStructures(cw ChunkWriter, od *OverworldDensity, cx, cz int32, seed in
 			if originCx%spacing == 0 && originCz%spacing == 0 {
 				t := GetTemplate("plains_small_house_1")
 				if t != nil {
-					baseY := -64 // MinY
-					if dx == 0 && dz == 0 {
-						baseY += surfTop[8][8] + 1
-					} else {
-						// sample height at center of origin chunk
-						_ = SampleColumn2D(od, 63, originCx*16+8, originCz*16+8)
-						// We don't have a fast exact heightmap lookup without running the column,
-						// but this is acceptable for a quick prototype.
-						baseY = 70 
+					// Deterministically evaluate the surface height at the origin so the structure 
+					// is flush with the ground and consistent across all chunks.
+					baseY := EvaluateHeight(od, originCx*16+8, originCz*16+8)
+					
+					// Only place if it's above sea level (so we don't spawn houses in the ocean)
+					if baseY >= 63 {
+						// Offset by -1 so the floor embeds into the grass, rather than floating on it.
+						t.Place(cw, cx, cz, originCx*16+8, baseY-1, originCz*16+8)
 					}
-					t.Place(cw, cx, cz, originCx*16+8, baseY, originCz*16+8)
 				}
 			}
 		}
 	}
 }
+
+// EvaluateHeight computes the top solid block Y coordinate at (wx, wz).
+func EvaluateHeight(od *OverworldDensity, wx, wz int) int {
+	interp := make([]float64, len(od.Interpolated))
+	
+	// Binary search or linear scan. A linear scan from 120 down to 0 is fast enough.
+	for y := 120; y >= -64; y-- {
+		ctx := FunctionContext{X: float64(wx), Y: float64(y), Z: float64(wz)}
+		// Since we don't have the cell grid, we just evaluate the interpolated nodes directly!
+		for n, node := range od.Interpolated {
+			interp[n] = node.Inner.Compute(ctx)
+		}
+		ctx = ctx.WithInterp(interp)
+		
+		if od.Final.Compute(ctx) > 0 {
+			return y
+		}
+	}
+	return 64
+}