Add Calendar: in-game days layered over GameClock

Introduce a Calendar in Core that turns scaled clock time into whole
in-game days plus a fraction-of-day, with a configurable SecondsPerDay.
Pausing or changing TimeScale slows or stops it automatically. Pure
read-side and deterministic, registered via context.UseCalendar(...),
mirroring GameSpeed. Covered by xUnit tests; docs updated.

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

tests/MrGameEng.Core.Tests/CalendarTests.cs

@@ -0,0 +1,69 @@
+using MrGameEng.Core;
+using Xunit;
+
+namespace MrGameEng.Core.Tests;
+
+public class CalendarTests
+{
+    [Fact]
+    public void Constructor_NonPositiveSecondsPerDay_Throws()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => new Calendar(new GameClock(), 0f));
+    }
+
+    [Fact]
+    public void TotalDays_TracksScaledClockTime()
+    {
+        var clock = new GameClock();
+        var calendar = new Calendar(clock, secondsPerDay: 10f);
+
+        clock.Advance(5f); // half a day
+
+        Assert.Equal(0.5, calendar.TotalDays, 5);
+        Assert.Equal(1, calendar.Day);
+        Assert.Equal(0.5f, calendar.DayProgress, 5);
+    }
+
+    [Fact]
+    public void Day_CountsFromOne_AndRollsOver()
+    {
+        var clock = new GameClock();
+        var calendar = new Calendar(clock, secondsPerDay: 10f);
+
+        Assert.Equal(1, calendar.Day);
+
+        clock.Advance(10f); // exactly one day
+        Assert.Equal(2, calendar.Day);
+        Assert.Equal(0f, calendar.DayProgress, 5);
+
+        clock.Advance(15f); // total 25s = 2.5 days
+        Assert.Equal(3, calendar.Day);
+        Assert.Equal(0.5f, calendar.DayProgress, 5);
+    }
+
+    [Fact]
+    public void Pause_DoesNotAdvanceTheCalendar()
+    {
+        var clock = new GameClock { TimeScale = 0f };
+        var calendar = new Calendar(clock, secondsPerDay: 10f);
+
+        clock.Advance(100f);
+
+        Assert.Equal(0.0, calendar.TotalDays, 5);
+        Assert.Equal(1, calendar.Day);
+    }
+
+    [Fact]
+    public void TimeScale_SpeedsUpTheCalendar()
+    {
+        var slow = new GameClock();
+        var fast = new GameClock { TimeScale = 6f };
+        var slowCal = new Calendar(slow, secondsPerDay: 10f);
+        var fastCal = new Calendar(fast, secondsPerDay: 10f);
+
+        slow.Advance(10f);
+        fast.Advance(10f);
+
+        Assert.Equal(6.0, fastCal.TotalDays / slowCal.TotalDays, 5);
+    }
+}