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>

src/MrGameEng.Core/Climate.cs

@@ -0,0 +1,137 @@
+namespace MrGameEng.Core;
+
+/// <summary>The four seasons, in calendar order from the start of the year.</summary>
+public enum Season
+{
+    /// <summary>First quarter of the year — warming.</summary>
+    Spring,
+
+    /// <summary>Second quarter — warmest.</summary>
+    Summer,
+
+    /// <summary>Third quarter — cooling.</summary>
+    Autumn,
+
+    /// <summary>Last quarter — coldest.</summary>
+    Winter,
+}
+
+/// <summary>
+/// Tuning for <see cref="Climate"/>. A year spans <see cref="DaysPerYear"/> in-game days; temperature
+/// follows a seasonal cosine peaking on <see cref="WarmestDay"/>, plus a daily swing (cooler at night).
+/// </summary>
+public readonly record struct ClimateSettings
+{
+    /// <summary>In-game days in one year (e.g. 60 = four 15-day seasons).</summary>
+    public int DaysPerYear { get; init; }
+
+    /// <summary>Yearly mean temperature.</summary>
+    public float MeanTemperature { get; init; }
+
+    /// <summary>Peak deviation from the mean across the seasons (summer high / winter low).</summary>
+    public float SeasonalAmplitude { get; init; }
+
+    /// <summary>Peak deviation from the seasonal mean across one day (warmer at noon, cooler at night).</summary>
+    public float DailyAmplitude { get; init; }
+
+    /// <summary>Day of the year (0-based) with the highest temperature; defaults to mid-summer.</summary>
+    public int WarmestDay { get; init; }
+
+    /// <summary>Temperate defaults: a 60-day year, mean 12°, ±14° seasonal, ±5° daily, warmest mid-summer.</summary>
+    public static ClimateSettings Default =>
+        new()
+        {
+            DaysPerYear = 60,
+            MeanTemperature = 12f,
+            SeasonalAmplitude = 14f,
+            DailyAmplitude = 5f,
+            WarmestDay = 22, // ~middle of the summer quarter of a 60-day year
+        };
+}
+
+/// <summary>
+/// Continuous climate layered over <see cref="Calendar"/>: a temperature that varies smoothly with the
+/// season and the time of day, plus the current season and year. The whole curve is derived from the
+/// calendar's elapsed time, so it slows or stops with the game clock. Pure read-side, GPU-free and
+/// deterministic; registered as a service via <see cref="ClimateEngineExtensions.UseClimate"/>.
+/// </summary>
+public sealed class Climate
+{
+    private readonly Calendar _calendar;
+    private readonly ClimateSettings _settings;
+
+    /// <summary>Creates a climate reading <paramref name="calendar"/> with the given <paramref name="settings"/>.</summary>
+    public Climate(Calendar calendar, ClimateSettings settings)
+    {
+        _calendar = calendar ?? throw new ArgumentNullException(nameof(calendar));
+        if (settings.DaysPerYear <= 0)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(settings),
+                "Days per year must be positive."
+            );
+        }
+
+        _settings = settings;
+    }
+
+    /// <summary>In-game days per year.</summary>
+    public int DaysPerYear => _settings.DaysPerYear;
+
+    /// <summary>Continuous position within the current year in <c>[0, 1)</c>.</summary>
+    public double YearProgress
+    {
+        get
+        {
+            var years = _calendar.TotalDays / _settings.DaysPerYear;
+            return years - Math.Floor(years);
+        }
+    }
+
+    /// <summary>The current year, counting from 1.</summary>
+    public int Year => (int)(_calendar.TotalDays / _settings.DaysPerYear) + 1;
+
+    /// <summary>Day within the current year, 0-based.</summary>
+    public int DayOfYear => (int)(YearProgress * _settings.DaysPerYear);
+
+    /// <summary>The current season, derived from the quarter of the year.</summary>
+    public Season Season => (Season)Math.Clamp((int)(YearProgress * 4.0), 0, 3);
+
+    /// <summary>Current temperature: seasonal cosine peaking on the warmest day, plus a daily swing.</summary>
+    public float Temperature
+    {
+        get
+        {
+            var warmFraction = _settings.WarmestDay / (float)_settings.DaysPerYear;
+            var seasonal =
+                _settings.MeanTemperature
+                + _settings.SeasonalAmplitude
+                    * MathF.Cos(MathF.Tau * ((float)YearProgress - warmFraction));
+            var daily = _settings.DailyAmplitude * (2f * Daylight() - 1f);
+            return seasonal + daily;
+        }
+    }
+
+    // Daytime factor 0..1 (0 at midnight, 1 at noon). Mirrors the day/night curve without a
+    // Graphics dependency — Core owns the daily temperature swing.
+    private float Daylight()
+    {
+        var value = -MathF.Cos(MathF.Tau * _calendar.DayProgress);
+        return value > 0f ? value : 0f;
+    }
+}
+
+/// <summary>Wires the climate model into the engine.</summary>
+public static class ClimateEngineExtensions
+{
+    /// <summary>
+    /// Creates a <see cref="Climate"/> bound to the context's <see cref="Calendar"/> service and
+    /// registers it. Call once per world, after <see cref="CalendarEngineExtensions.UseCalendar"/>.
+    /// </summary>
+    public static Climate UseClimate(this EngineContext context, ClimateSettings settings)
+    {
+        var climate = new Climate(context.Services.Get<Calendar>(), settings);
+        context.Services.Add(climate);
+        return climate;
+    }
+}