Initial commit: Theriapolis baseline at port/godot branch point

Captures the pre-Godot-port state of the codebase. This is the rollback
anchor for the Godot port (M0 of theriapolis-rpg-implementation-plan-godot-port.md).
All Phase 0 through Phase 6.5 work is included; Phase 7 is in flight.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Theriapolis.Tests/Entities/WorldClockTests.cs

@@ -0,0 +1,54 @@
+using Theriapolis.Core.Time;
+using Xunit;
+
+namespace Theriapolis.Tests.Entities;
+
+public sealed class WorldClockTests
+{
+    [Fact]
+    public void Advance_AccumulatesSeconds()
+    {
+        var c = new WorldClock();
+        c.Advance(120);
+        c.Advance(30);
+        Assert.Equal(150, c.InGameSeconds);
+    }
+
+    [Fact]
+    public void DayHourMinute_DerivedFromSeconds()
+    {
+        var c = new WorldClock();
+        c.Advance(WorldClock.SecondsPerDay * 3 + WorldClock.SecondsPerHour * 14 + WorldClock.SecondsPerMinute * 27);
+        Assert.Equal(3, c.Day);
+        Assert.Equal(14, c.Hour);
+        Assert.Equal(27, c.Minute);
+    }
+
+    [Fact]
+    public void Season_RotatesWithDays()
+    {
+        var c = new WorldClock();
+        // Advance past one full season (24 days).
+        c.Advance(WorldClock.SecondsPerDay * WorldClock.DaysPerSeason + 1);
+        Assert.Equal(Season.Summer, c.Season);
+    }
+
+    [Fact]
+    public void RoundTrip_RestoresState()
+    {
+        var c = new WorldClock();
+        c.Advance(98765);
+        var s = c.CaptureState();
+
+        var c2 = new WorldClock();
+        c2.RestoreState(s);
+        Assert.Equal(98765, c2.InGameSeconds);
+    }
+
+    [Fact]
+    public void Advance_RejectsNegative()
+    {
+        var c = new WorldClock();
+        Assert.Throws<ArgumentOutOfRangeException>(() => c.Advance(-5));
+    }
+}