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>

src/AcDream.Core/Plugins/WorldEvents.cs

@@ -0,0 +1,56 @@
+// src/AcDream.Core/Plugins/WorldEvents.cs
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Plugins;
+
+public sealed class WorldEvents : IEvents
+{
+    private readonly object _lock = new();
+    private readonly List<WorldEntitySnapshot> _alreadySpawned = new();
+    private Action<WorldEntitySnapshot>? _subscribers;
+
+    /// <summary>
+    /// Called by the host as each entity is hydrated into the world. Records the
+    /// snapshot for later replay and dispatches to current subscribers.
+    /// </summary>
+    public void FireEntitySpawned(WorldEntitySnapshot snapshot)
+    {
+        Action<WorldEntitySnapshot>? toNotify;
+        lock (_lock)
+        {
+            _alreadySpawned.Add(snapshot);
+            toNotify = _subscribers;
+        }
+
+        if (toNotify is null) return;
+        foreach (Action<WorldEntitySnapshot> handler in toNotify.GetInvocationList())
+        {
+            try { handler(snapshot); }
+            catch { /* plugin errors don't propagate out of event dispatch */ }
+        }
+    }
+
+    public event Action<WorldEntitySnapshot> EntitySpawned
+    {
+        add
+        {
+            WorldEntitySnapshot[] replay;
+            lock (_lock)
+            {
+                _subscribers += value;
+                replay = _alreadySpawned.ToArray();
+            }
+            // Replay outside the lock to avoid deadlock if a handler re-enters.
+            foreach (var s in replay)
+            {
+                try { value(s); }
+                catch { /* plugin errors don't propagate out of += */ }
+            }
+        }
+        remove
+        {
+            lock (_lock)
+                _subscribers -= value;
+        }
+    }
+}

src/AcDream.Core/Plugins/WorldGameState.cs

@@ -0,0 +1,14 @@
+// src/AcDream.Core/Plugins/WorldGameState.cs
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Plugins;
+
+public sealed class WorldGameState : IGameState
+{
+    private readonly List<WorldEntitySnapshot> _entities = new();
+
+    public IReadOnlyList<WorldEntitySnapshot> Entities => _entities;
+
+    /// <summary>Called by the host as each entity is hydrated.</summary>
+    public void Add(WorldEntitySnapshot snapshot) => _entities.Add(snapshot);
+}