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

src/TheLivingWorld.Core/Geo/LocalProjection.cs

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+using System.Numerics;
+
+namespace TheLivingWorld.Core.Geo;
+
+/// <summary>
+/// Flattens WGS84 coordinates onto a local metric plane centred on an origin: X grows east, Y grows north,
+/// both in metres. This is an equirectangular projection with the scale factor frozen at the origin latitude,
+/// which stays sub-metre accurate across the 5-20 km worlds this game generates and — unlike Web Mercator —
+/// keeps distances usable directly as game units.
+/// </summary>
+public sealed class LocalProjection
+{
+    public const double EarthRadiusMeters = 6378137.0;
+    public const double MetersPerDegreeLatitude = EarthRadiusMeters * Math.PI / 180.0;
+
+    private readonly double _metersPerDegreeLongitude;
+
+    public LocalProjection(GeoPoint origin)
+    {
+        if (!origin.IsValid)
+            throw new ArgumentOutOfRangeException(nameof(origin), origin, "Origin is not a valid WGS84 coordinate.");
+
+        Origin = origin;
+        var cosLat = Math.Cos(origin.Latitude * Math.PI / 180.0);
+        _metersPerDegreeLongitude = MetersPerDegreeLatitude * Math.Max(cosLat, 1e-6);
+    }
+
+    public GeoPoint Origin { get; }
+
+    public Vector2 Project(GeoPoint point) => Project(point.Latitude, point.Longitude);
+
+    public Vector2 Project(double latitude, double longitude) => new(
+        (float)((longitude - Origin.Longitude) * _metersPerDegreeLongitude),
+        (float)((latitude - Origin.Latitude) * MetersPerDegreeLatitude));
+
+    public GeoPoint Unproject(Vector2 local) => new(
+        Origin.Latitude + local.Y / MetersPerDegreeLatitude,
+        Origin.Longitude + local.X / _metersPerDegreeLongitude);
+}