Add Climate and day/night ambient lighting

Climate (Core) layers a continuous seasonal-plus-daily temperature and the
current season over the Calendar, registered via context.UseClimate. A new
Lighting module (Graphics) adds DayNight: a daylight factor and ambient
color from the calendar's time of day, pushed into the renderer's new
AmbientLight (multiplied into world-space sprites only, so the scene
darkens at night while screen-space overlays stay readable) and exposed as
a sampleable SampleAt for the simulation. Both deterministic and GPU-free,
covered by tests; docs updated.

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

tests/MrGameEng.Graphics.Tests/DayNightTests.cs

@@ -0,0 +1,54 @@
+using MrGameEng.Core;
+using MrGameEng.Lighting;
+using Xunit;
+
+namespace MrGameEng.Lighting.Tests;
+
+public class DayNightTests
+{
+    private static (GameClock Clock, DayNight DayNight) Make()
+    {
+        var clock = new GameClock();
+        var calendar = new Calendar(clock, secondsPerDay: 24f); // 1 second = 1 in-game hour
+        return (clock, new DayNight(calendar, DayNightSettings.Default));
+    }
+
+    [Fact]
+    public void Daylight_ZeroAtMidnight_OneAtNoon()
+    {
+        var (clock, dayNight) = Make();
+
+        Assert.Equal(0f, dayNight.Daylight, 3); // 00:00
+
+        clock.Advance(12f); // 12:00
+        Assert.Equal(1f, dayNight.Daylight, 3);
+    }
+
+    [Fact]
+    public void Daylight_RisesFromDawnToNoon()
+    {
+        var (clock, dayNight) = Make();
+
+        clock.Advance(6f); // 06:00
+        var dawn = dayNight.Daylight;
+        clock.Advance(3f); // 09:00
+        var mid = dayNight.Daylight;
+        clock.Advance(3f); // 12:00
+        var noon = dayNight.Daylight;
+
+        Assert.True(dawn < mid && mid < noon, "daylight should rise from dawn to noon");
+    }
+
+    [Fact]
+    public void Ambient_DarkerAtNightThanAtNoon()
+    {
+        var (clock, dayNight) = Make();
+
+        var night = dayNight.Ambient; // 00:00
+        clock.Advance(12f); // 12:00
+        var noon = dayNight.Ambient;
+
+        Assert.True(night.R < noon.R && night.G < noon.G && night.B < noon.B);
+        Assert.Equal(255, noon.R); // day color is white at noon
+    }
+}