Add 2D lightmap: occlusion shadows and point lights

Extend the Lighting module with a per-cell lightmap. LightmapBuilder (pure,
tested) fills an ambient base, shades occluder cells, and adds point lights
that attenuate with distance and are blocked by occluders between source and
cell (grid-traced soft shadows). Lightmap holds the grid plus a greyscale
texture (Upload) and a bilinear SampleAt for the simulation; a PointLight
component places lights in the world. LightmapSystem rebuilds the grid a few
times a second (ambient from the day/night cycle with a night floor, occluders
from a game-supplied grid, point lights from the ECS); LightmapRenderSystem
multiplies the lightmap over the world after the sprite flush and under the
HUD. Wired via scene.UseLighting(...), sampled through Lighting.SampleAt. Docs
updated.

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

tests/MrGameEng.Graphics.Tests/LightmapBuilderTests.cs

@@ -0,0 +1,69 @@
+using MrGameEng.Lighting;
+using Xunit;
+
+namespace MrGameEng.Lighting.Tests;
+
+public class LightmapBuilderTests
+{
+    private static float[] Build(
+        int width,
+        int height,
+        float ambient,
+        bool[] occluders,
+        params LightSample[] lights
+    )
+    {
+        var light = new float[width * height];
+        LightmapBuilder.Build(light, width, height, ambient, occluders, lights);
+        return light;
+    }
+
+    [Fact]
+    public void Ambient_WithoutOccluders_IsUniform()
+    {
+        var light = Build(4, 4, 0.6f, new bool[16]);
+        Assert.All(light, v => Assert.Equal(0.6f, v, 5));
+    }
+
+    [Fact]
+    public void OccluderCell_IsDarkerThanOpenCell()
+    {
+        var occ = new bool[9];
+        occ[4] = true; // centre cell shaded
+        var light = Build(3, 3, 0.8f, occ);
+        Assert.True(light[4] < light[0]);
+        Assert.Equal(0.8f * LightmapBuilder.OccluderShade, light[4], 5);
+    }
+
+    [Fact]
+    public void PointLight_IsBrighterNearSourceAndFadesToRadius()
+    {
+        var light = Build(7, 1, 0f, new bool[7], new LightSample(0, 0, 6f, 1f));
+        Assert.True(light[0] > light[2] && light[2] > light[5]);
+        Assert.InRange(light[6], 0f, 0.01f); // на радиусе свет угасает
+    }
+
+    [Fact]
+    public void Occluder_CastsShadowBehindIt()
+    {
+        var occ = new bool[7];
+        occ[3] = true; // препятствие между источником (0) и дальними клетками
+        var light = Build(7, 1, 0f, occ, new LightSample(0, 0, 10f, 1f));
+
+        Assert.True(light[1] > 0f); // перед препятствием — освещено
+        Assert.Equal(0f, light[5], 5); // за препятствием — тень
+    }
+
+    [Fact]
+    public void Values_StayWithinUnitRange()
+    {
+        var light = Build(
+            5,
+            5,
+            0.5f,
+            new bool[25],
+            new LightSample(2, 2, 4f, 2f) // яркий свет — проверяем клампинг
+        );
+        Assert.All(light, v => Assert.InRange(v, 0f, 1f));
+    }
+}