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>

src/MrGameEng.Graphics/Lighting/PointLight.cs

@@ -0,0 +1,21 @@
+using Friflo.Engine.ECS;
+using Microsoft.Xna.Framework;
+
+namespace MrGameEng.Lighting;
+
+/// <summary>
+/// A point light: an entity with this component plus a <see cref="MrGameEng.Graphics.Transform2D"/>
+/// adds light around its world position, attenuating to zero at <see cref="Radius"/> and casting
+/// grid-traced shadows behind occluders. Picked up by the lighting system into the lightmap.
+/// </summary>
+public struct PointLight : IComponent
+{
+    /// <summary>Reach of the light in world units (light fades to zero at this distance).</summary>
+    public float Radius;
+
+    /// <summary>Light tint (reserved for colored lights; intensity currently drives brightness).</summary>
+    public Color Color;
+
+    /// <summary>Peak brightness added at the light's centre (0..1+).</summary>
+    public float Intensity;
+}