Add MrGameEng.WorldGen: deterministic procedural world generation

Introduce a WorldGen module under Simulation with reusable helpers for
seed-based terrain heightmaps: PerlinNoise (deterministic 2D gradient
noise), FractalNoise (fBm over octaves), Falloff (island edge masks),
Heightmap (row-major grid with min-max normalize) and HeightmapGenerator
(settings to normalized heightmap). All deterministic from an integer
seed, allocation-free per sample, covered by xUnit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/MrGameEng.Simulation/WorldGen/Heightmap.cs

@@ -0,0 +1,64 @@
+namespace MrGameEng.WorldGen;
+
+/// <summary>
+/// A dense row-major grid of height values. Plain data with indexed access — fill it from a
+/// <see cref="HeightmapGenerator"/> and read it from worldgen code. Values are arbitrary until
+/// <see cref="Normalize"/> rescales them into <c>[0, 1]</c>.
+/// </summary>
+public sealed class Heightmap
+{
+    /// <summary>Grid width in cells.</summary>
+    public int Width { get; }
+
+    /// <summary>Grid height in cells.</summary>
+    public int Height { get; }
+
+    /// <summary>Row-major values, length <see cref="Width"/>×<see cref="Height"/>.</summary>
+    public float[] Values { get; }
+
+    /// <summary>Creates an all-zero map of the given size.</summary>
+    public Heightmap(int width, int height)
+    {
+        Width = width;
+        Height = height;
+        Values = new float[width * height];
+    }
+
+    /// <summary>Height at (<paramref name="x"/>, <paramref name="y"/>).</summary>
+    public float this[int x, int y]
+    {
+        get => Values[y * Width + x];
+        set => Values[y * Width + x] = value;
+    }
+
+    /// <summary>Rescales all values into <c>[0, 1]</c> by min–max; a flat map becomes all zeros.</summary>
+    public void Normalize()
+    {
+        var min = float.MaxValue;
+        var max = float.MinValue;
+        foreach (var v in Values)
+        {
+            if (v < min)
+            {
+                min = v;
+            }
+
+            if (v > max)
+            {
+                max = v;
+            }
+        }
+
+        var range = max - min;
+        if (range <= float.Epsilon)
+        {
+            Array.Clear(Values);
+            return;
+        }
+
+        for (var i = 0; i < Values.Length; i++)
+        {
+            Values[i] = (Values[i] - min) / range;
+        }
+    }
+}