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/Determinism/SeededRngTests.cs

@@ -0,0 +1,77 @@
+using Theriapolis.Core.Util;
+using Xunit;
+
+namespace Theriapolis.Tests.Determinism;
+
+/// <summary>
+/// Phase 0 smoke tests: two SeededRng instances with the same seed must produce
+/// identical outputs, and different seeds must diverge.
+/// </summary>
+public sealed class SeededRngTests
+{
+    [Fact]
+    public void SameSeed_ProducesSameSequence()
+    {
+        var a = new SeededRng(123);
+        var b = new SeededRng(123);
+
+        for (int i = 0; i < 1000; i++)
+            Assert.Equal(a.NextUInt64(), b.NextUInt64());
+    }
+
+    [Fact]
+    public void DifferentSeeds_ProduceDifferentSequences()
+    {
+        var a = new SeededRng(123);
+        var b = new SeededRng(456);
+
+        bool anyDifferent = false;
+        for (int i = 0; i < 10; i++)
+            if (a.NextUInt64() != b.NextUInt64()) { anyDifferent = true; break; }
+
+        Assert.True(anyDifferent, "Different seeds should produce different sequences.");
+    }
+
+    [Fact]
+    public void ZeroSeed_DoesNotGetStuck()
+    {
+        var rng = new SeededRng(0);
+        ulong prev = rng.NextUInt64();
+        bool moved = false;
+        for (int i = 0; i < 5; i++)
+        {
+            ulong next = rng.NextUInt64();
+            if (next != prev) { moved = true; break; }
+            prev = next;
+        }
+        Assert.True(moved, "RNG must advance even from seed 0.");
+    }
+
+    [Fact]
+    public void NextFloat_InRange()
+    {
+        var rng = new SeededRng(999);
+        for (int i = 0; i < 10_000; i++)
+        {
+            float f = rng.NextFloat();
+            Assert.True(f >= 0f && f < 1f, $"float {f} out of [0,1)");
+        }
+    }
+
+    [Fact]
+    public void ForSubsystem_DifferentTagsProduceDifferentStreams()
+    {
+        var a = SeededRng.ForSubsystem(0xDEADBEEF, Theriapolis.Core.C.RNG_TERRAIN);
+        var b = SeededRng.ForSubsystem(0xDEADBEEF, Theriapolis.Core.C.RNG_MOISTURE);
+        Assert.NotEqual(a.NextUInt64(), b.NextUInt64());
+    }
+
+    [Fact]
+    public void Seed123_FirstValue_IsReproducible()
+    {
+        // Two independent instances must produce the exact same first value.
+        var x = new SeededRng(123).NextUInt64();
+        var y = new SeededRng(123).NextUInt64();
+        Assert.Equal(x, y);
+    }
+}