Complete portable planetary presentation contracts

Tools~/CultMath.PlanetaryTileFixture/Program.cs

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+using CultMath;
+
+if (args.Length != 1) throw new ArgumentException("Expected one output path.");
+var field = PlanetaryFieldDefinition.Create(0x55aa, 10, 7, AdvancedErosionParameters.Default);
+var layout = new PlanetaryMapTileLayout(new(PlanetaryProjectionKind.EqualEarth), 0, 0, 0, 5, 1);
+var key = new PlanetaryMapTileKey(field.FieldVersion, 3, 29, layout, PlanetaryQueryScale.AtFootprint(0.25f, 0.125f));
+var tile = PlanetaryMapTileBaker.Bake(field, key, new FixtureField());
+var path = Path.GetFullPath(args[0]);
+Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+using var stream = File.Create(path);
+PlanetaryMapTileEncoding.Write(stream, tile);
+
+readonly struct FixtureField : IPlanetaryBaseField
+{
+    public PlanetaryBaseFieldSample Sample(float3 direction)
+    {
+        var value = direction.x * 0.2f + direction.z * 0.4f;
+        var gradient = new float3(0.2f, 0, 0.4f);
+        gradient -= direction * math.dot(gradient, direction);
+        return new(value, gradient, value, gradient * 0.1f, value);
+    }
+}