Update README with project details and setup instructions; add data directories to .gitignore

tests/TheLivingWorld.Tests/GeometryTests.cs

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+using System.Numerics;
+using TheLivingWorld.Core.Geo;
+using TheLivingWorld.Osm.Import;
+
+namespace TheLivingWorld.Tests;
+
+public class GeometryClipperTests
+{
+    private static readonly RectBounds Unit = new(0, 0, 100, 100);
+
+    [Fact]
+    public void Leaves_a_polygon_that_is_already_inside_alone()
+    {
+        Vector2[] square = [new(10, 10), new(90, 10), new(90, 90), new(10, 90)];
+
+        var clipped = GeometryClipper.ClipPolygon(square, Unit);
+
+        Assert.NotNull(clipped);
+        Assert.Equal(4, clipped.Length);
+        Assert.Equal(Polygons.Area(square), Polygons.Area(clipped), 1);
+    }
+
+    [Fact]
+    public void Trims_a_polygon_that_hangs_over_the_edge()
+    {
+        Vector2[] overhanging = [new(50, 50), new(200, 50), new(200, 90), new(50, 90)];
+
+        var clipped = GeometryClipper.ClipPolygon(overhanging, Unit);
+
+        Assert.NotNull(clipped);
+        Assert.All(clipped, point => Assert.InRange(point.X, 0, 100));
+        Assert.Equal(50f * 40f, Polygons.Area(clipped), 1);
+    }
+
+    [Fact]
+    public void Drops_a_polygon_that_misses_the_box_entirely()
+    {
+        Vector2[] elsewhere = [new(500, 500), new(600, 500), new(600, 600)];
+
+        Assert.Null(GeometryClipper.ClipPolygon(elsewhere, Unit));
+    }
+
+    [Fact]
+    public void Keeps_the_part_of_a_line_that_crosses_the_box()
+    {
+        Vector2[] line = [new(-50, 50), new(150, 50)];
+
+        var runs = GeometryClipper.ClipPolyline(line, Unit);
+
+        var run = Assert.Single(runs);
+        Assert.Equal(new Vector2(0, 50), run[0]);
+        Assert.Equal(new Vector2(100, 50), run[^1]);
+    }
+
+    [Fact]
+    public void Splits_a_line_that_leaves_and_comes_back()
+    {
+        Vector2[] line = [new(10, 50), new(150, 50), new(150, 20), new(10, 20)];
+
+        var runs = GeometryClipper.ClipPolyline(line, Unit);
+
+        Assert.Equal(2, runs.Count);
+        Assert.All(runs, run => Assert.All(run, point => Assert.InRange(point.X, 0, 100)));
+    }
+
+    [Fact]
+    public void Drops_a_line_that_never_touches_the_box()
+    {
+        Vector2[] line = [new(-50, -50), new(-10, -20)];
+
+        Assert.Empty(GeometryClipper.ClipPolyline(line, Unit));
+    }
+}
+
+public class PolygonsTests
+{
+    [Fact]
+    public void Computes_the_area_of_a_square()
+    {
+        Vector2[] square = [new(0, 0), new(10, 0), new(10, 10), new(0, 10)];
+
+        Assert.Equal(100f, Polygons.Area(square));
+    }
+
+    [Fact]
+    public void Detects_containment()
+    {
+        Vector2[] square = [new(0, 0), new(10, 0), new(10, 10), new(0, 10)];
+
+        Assert.True(Polygons.Contains(square, new Vector2(5, 5)));
+        Assert.False(Polygons.Contains(square, new Vector2(15, 5)));
+    }
+}
+
+public class RingAssemblerTests
+{
+    [Fact]
+    public void Joins_fragments_into_one_ring_whatever_their_order_or_direction()
+    {
+        // A 10x10 square split into three fragments; the middle one runs backwards.
+        List<IReadOnlyList<Vector2>> fragments =
+        [
+            [new Vector2(10, 10), new Vector2(0, 10), new Vector2(0, 0)],
+            [new Vector2(0, 0), new Vector2(10, 0)],
+            [new Vector2(10, 10), new Vector2(10, 0)],
+        ];
+
+        var rings = RingAssembler.Assemble(fragments);
+
+        var ring = Assert.Single(rings);
+        Assert.Equal(4, ring.Length);
+        Assert.Equal(100f, Polygons.Area(ring));
+    }
+
+    [Fact]
+    public void Accepts_a_way_that_is_already_closed()
+    {
+        List<IReadOnlyList<Vector2>> fragments =
+        [
+            [new Vector2(0, 0), new Vector2(10, 0), new Vector2(10, 10), new Vector2(0, 10), new Vector2(0, 0)],
+        ];
+
+        var ring = Assert.Single(RingAssembler.Assemble(fragments));
+
+        // The repeated closing vertex is dropped: a ring is stored open.
+        Assert.Equal(4, ring.Length);
+    }
+
+    [Fact]
+    public void Drops_fragments_that_never_close()
+    {
+        List<IReadOnlyList<Vector2>> fragments =
+        [
+            [new Vector2(0, 0), new Vector2(10, 0)],
+            [new Vector2(10, 0), new Vector2(10, 10)],
+        ];
+
+        Assert.Empty(RingAssembler.Assemble(fragments));
+    }
+
+    [Fact]
+    public void Separates_two_independent_rings()
+    {
+        List<IReadOnlyList<Vector2>> fragments =
+        [
+            [new Vector2(0, 0), new Vector2(10, 0), new Vector2(10, 10), new Vector2(0, 10), new Vector2(0, 0)],
+            [new Vector2(50, 50), new Vector2(60, 50), new Vector2(60, 60), new Vector2(50, 50)],
+        ];
+
+        Assert.Equal(2, RingAssembler.Assemble(fragments).Count);
+    }
+}