Enhance Calendar and Climate systems with start day offsets

- Added a `startDay` parameter to the `Calendar` class to allow for fractional day offsets at clock initialization, enabling more flexible time settings.
- Updated `TotalDays` calculation to include the `startDay` offset.
- Introduced `StartDayOfYear` in `ClimateSettings` to shift the seasonal phase, allowing worlds to begin at a specific time of year.
- Adjusted `YearProgress` and `Year` calculations in the `Climate` class to account for the new `StartDayOfYear`.
- Added unit tests to verify the functionality of the new start day features in both `Calendar` and `Climate` classes.

tests/MrGameEng.Core.Tests/ClimateTests.cs

@@ -81,4 +81,15 @@ public class ClimateTests
         Assert.Equal(2, climate.Year);
         Assert.Equal(0, climate.DayOfYear);
     }
+
+    [Fact]
+    public void StartDayOfYear_ShiftsTheSeasonalPhase()
+    {
+        // Begin the world already at the warmest day (15): the curve peaks at clock 0.
+        var (_, climate) = Make(Seasonal with { StartDayOfYear = 15 });
+
+        Assert.Equal(30f, climate.Temperature, 2); // mean 10 + amplitude 20, at the peak
+        Assert.Equal(15, climate.DayOfYear); // day-of-year reflects the offset
+        Assert.Equal(Season.Summer, climate.Season); // day 15 of a 60-day year = start of summer
+    }
 }