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.Core.Tests/ClimateTests.cs

@@ -0,0 +1,84 @@
+using MrGameEng.Core;
+using Xunit;
+
+namespace MrGameEng.Core.Tests;
+
+public class ClimateTests
+{
+    private static (GameClock Clock, Climate Climate) Make(ClimateSettings settings)
+    {
+        var clock = new GameClock();
+        var calendar = new Calendar(clock, secondsPerDay: 10f);
+        return (clock, new Climate(calendar, settings));
+    }
+
+    private static ClimateSettings Seasonal =>
+        new()
+        {
+            DaysPerYear = 60,
+            MeanTemperature = 10f,
+            SeasonalAmplitude = 20f,
+            DailyAmplitude = 0f,
+            WarmestDay = 15,
+        };
+
+    [Fact]
+    public void Constructor_NonPositiveYear_Throws()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() =>
+            new Climate(new Calendar(new GameClock(), 10f), Seasonal with { DaysPerYear = 0 })
+        );
+    }
+
+    [Fact]
+    public void Temperature_PeaksOnWarmestDay_AndBottomsHalfYearLater()
+    {
+        var (clock, climate) = Make(Seasonal);
+
+        clock.Advance(15 * 10f); // day 15 = warmest
+        Assert.Equal(30f, climate.Temperature, 2); // mean 10 + amplitude 20
+
+        clock.Advance(30 * 10f); // +30 days = half a 60-day year later
+        Assert.Equal(-10f, climate.Temperature, 2); // mean 10 - amplitude 20
+    }
+
+    [Fact]
+    public void DailySwing_WarmerAtNoonThanMidnight()
+    {
+        var settings = Seasonal with { SeasonalAmplitude = 0f, DailyAmplitude = 6f };
+        var (clock, climate) = Make(settings);
+
+        clock.Advance(5f); // day 0, noon (DayProgress 0.5)
+        Assert.Equal(16f, climate.Temperature, 2); // mean 10 + daily 6
+
+        clock.Advance(5f); // day 1, midnight (DayProgress 0)
+        Assert.Equal(4f, climate.Temperature, 2); // mean 10 - daily 6
+    }
+
+    [Fact]
+    public void Season_FollowsTheQuarterOfTheYear()
+    {
+        var (clock, climate) = Make(Seasonal); // 60-day year → 15 days per season
+
+        Assert.Equal(Season.Spring, climate.Season);
+        clock.Advance(20 * 10f);
+        Assert.Equal(Season.Summer, climate.Season);
+        clock.Advance(15 * 10f);
+        Assert.Equal(Season.Autumn, climate.Season);
+        clock.Advance(15 * 10f);
+        Assert.Equal(Season.Winter, climate.Season);
+    }
+
+    [Fact]
+    public void YearAndDayOfYear_RollOver()
+    {
+        var (clock, climate) = Make(Seasonal);
+
+        Assert.Equal(1, climate.Year);
+        Assert.Equal(0, climate.DayOfYear);
+
+        clock.Advance(60 * 10f); // exactly one year
+        Assert.Equal(2, climate.Year);
+        Assert.Equal(0, climate.DayOfYear);
+    }
+}