feat(core): add IGameState, IEvents, WorldEvents with replay-on-subscribe

Adds WorldEntitySnapshot, IGameState, IEvents abstractions; WorldEvents
implements replay-on-subscribe with per-handler exception swallowing;
WorldGameState tracks entities; AppPluginHost exposes all three; stubs
wired in Program.cs to keep build green ahead of Task 9 live wiring.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Plugins/PluginLoaderTests.cs

@@ -28,6 +28,8 @@ public class PluginLoaderTests
     private sealed class StubHost : IPluginHost
     {
         public IPluginLogger Log { get; } = new StubLogger();
+        public IGameState State { get; } = new StubState();
+        public IEvents Events { get; } = new StubEvents();
     }
 
     private sealed class StubLogger : IPluginLogger
@@ -37,6 +39,20 @@ public class PluginLoaderTests
         public void Error(string message, Exception? exception = null) { }
     }
 
+    private sealed class StubState : IGameState
+    {
+        public IReadOnlyList<WorldEntitySnapshot> Entities { get; } = Array.Empty<WorldEntitySnapshot>();
+    }
+
+    private sealed class StubEvents : IEvents
+    {
+        public event Action<WorldEntitySnapshot> EntitySpawned
+        {
+            add { }
+            remove { }
+        }
+    }
+
     [Fact]
     public void Load_FixtureDll_InstantiatesPluginAndCallsInitialize()
     {

tests/AcDream.Core.Tests/Plugins/WorldEventsTests.cs

@@ -0,0 +1,87 @@
+// tests/AcDream.Core.Tests/Plugins/WorldEventsTests.cs
+using System.Numerics;
+using AcDream.Core.Plugins;
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Tests.Plugins;
+
+public class WorldEventsTests
+{
+    private static WorldEntitySnapshot S(uint id) => new(id, SourceId: 0x01000000u, Position: Vector3.Zero, Rotation: Quaternion.Identity);
+
+    [Fact]
+    public void FireBeforeAnySubscriber_LateSubscribeReceivesReplay()
+    {
+        var events = new WorldEvents();
+        events.FireEntitySpawned(S(1));
+        events.FireEntitySpawned(S(2));
+        events.FireEntitySpawned(S(3));
+
+        var seen = new List<uint>();
+        events.EntitySpawned += e => seen.Add(e.Id);
+
+        Assert.Equal(new uint[] { 1, 2, 3 }, seen);
+    }
+
+    [Fact]
+    public void FireAfterSubscribe_ReachesSubscriber()
+    {
+        var events = new WorldEvents();
+        var seen = new List<uint>();
+        events.EntitySpawned += e => seen.Add(e.Id);
+
+        events.FireEntitySpawned(S(10));
+        events.FireEntitySpawned(S(20));
+
+        Assert.Equal(new uint[] { 10, 20 }, seen);
+    }
+
+    [Fact]
+    public void ReplayPlusLive_DeliversExactlyOnceEach()
+    {
+        var events = new WorldEvents();
+        events.FireEntitySpawned(S(1));  // pre-subscribe
+
+        var seen = new List<uint>();
+        events.EntitySpawned += e => seen.Add(e.Id);  // replay fires 1
+
+        events.FireEntitySpawned(S(2));  // live fires 2
+
+        Assert.Equal(new uint[] { 1, 2 }, seen);
+    }
+
+    [Fact]
+    public void Unsubscribe_StopsLiveDelivery()
+    {
+        var events = new WorldEvents();
+        var seen = new List<uint>();
+        Action<WorldEntitySnapshot> handler = e => seen.Add(e.Id);
+
+        events.EntitySpawned += handler;
+        events.FireEntitySpawned(S(1));
+        events.EntitySpawned -= handler;
+        events.FireEntitySpawned(S(2));
+
+        Assert.Equal(new uint[] { 1 }, seen);
+    }
+
+    [Fact]
+    public void HandlerThrowsDuringReplay_OtherReplayEntriesStillDelivered()
+    {
+        var events = new WorldEvents();
+        events.FireEntitySpawned(S(1));
+        events.FireEntitySpawned(S(2));
+        events.FireEntitySpawned(S(3));
+
+        var seen = new List<uint>();
+        events.EntitySpawned += e =>
+        {
+            if (e.Id == 2) throw new InvalidOperationException("boom");
+            seen.Add(e.Id);
+        };
+
+        // No exception propagates out of the += add; 1 and 3 were still delivered.
+        Assert.Contains(1u, seen);
+        Assert.Contains(3u, seen);
+    }
+}