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/WorldCache.cs

@@ -0,0 +1,53 @@
+using Theriapolis.Core.World.Generation;
+
+namespace Theriapolis.Tests;
+
+/// <summary>
+/// Class-level fixture that memoizes worldgen pipeline runs so multiple tests
+/// in the same class (all hitting e.g. seed 0xCAFEBABE) share one expensive run.
+///
+/// Each test class gets its own WorldCache via <see cref="Xunit.IClassFixture{T}"/>,
+/// which keeps cross-class parallelism intact while collapsing within-class duplicate
+/// runs. A full pipeline takes ~30s; a test class that previously did 9 runs now does 1.
+///
+/// Cache key is (seed, stageThroughIndex, variant):
+///  - stageThroughIndex = -1 means RunAll; otherwise RunThrough(ctx, idx).
+///  - variant is used by determinism tests that intentionally want TWO independent
+///    runs of the same seed to compare: pass variant 0 and variant 1.
+/// </summary>
+public sealed class WorldCache : IDisposable
+{
+    private readonly Dictionary<(ulong Seed, int Stage, int Variant), WorldGenContext> _cache = new();
+    private readonly object _lock = new();
+
+    /// <summary>Full pipeline run for <paramref name="seed"/>, memoized.</summary>
+    public WorldGenContext Get(ulong seed, int variant = 0) =>
+        GetInternal(seed, stageThroughIndex: -1, variant);
+
+    /// <summary>
+    /// Partial pipeline run through stage index (0-based), memoized.
+    /// E.g. <c>GetThrough(seed, 9)</c> runs stages 1–10 (HydrologyGen).
+    /// </summary>
+    public WorldGenContext GetThrough(ulong seed, int stageThroughIndex, int variant = 0) =>
+        GetInternal(seed, stageThroughIndex, variant);
+
+    private WorldGenContext GetInternal(ulong seed, int stageThroughIndex, int variant)
+    {
+        var key = (seed, stageThroughIndex, variant);
+        lock (_lock)
+        {
+            if (_cache.TryGetValue(key, out var cached)) return cached;
+
+            var ctx = new WorldGenContext(seed, TestHelpers.DataDirectory);
+            if (stageThroughIndex < 0)
+                WorldGenerator.RunAll(ctx);
+            else
+                WorldGenerator.RunThrough(ctx, stageThroughIndex);
+
+            _cache[key] = ctx;
+            return ctx;
+        }
+    }
+
+    public void Dispose() => _cache.Clear();
+}