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.Plugin.Abstractions/IEvents.cs

@@ -0,0 +1,7 @@
+// src/AcDream.Plugin.Abstractions/IEvents.cs
+namespace AcDream.Plugin.Abstractions;
+
+public interface IEvents
+{
+    event Action<WorldEntitySnapshot> EntitySpawned;
+}

src/AcDream.Plugin.Abstractions/IGameState.cs

@@ -0,0 +1,7 @@
+// src/AcDream.Plugin.Abstractions/IGameState.cs
+namespace AcDream.Plugin.Abstractions;
+
+public interface IGameState
+{
+    IReadOnlyList<WorldEntitySnapshot> Entities { get; }
+}

src/AcDream.Plugin.Abstractions/IPluginHost.cs

@@ -3,9 +3,11 @@ namespace AcDream.Plugin.Abstractions;
 
 /// <summary>
 /// Entry point for a plugin into the acdream runtime. The surface will grow
-/// across phases as more systems come online. For Phase 1 only IPluginLogger is real.
+/// across phases as more systems come online.
 /// </summary>
 public interface IPluginHost
 {
     IPluginLogger Log { get; }
+    IGameState State { get; }
+    IEvents Events { get; }
 }

src/AcDream.Plugin.Abstractions/WorldEntitySnapshot.cs

@@ -0,0 +1,10 @@
+// src/AcDream.Plugin.Abstractions/WorldEntitySnapshot.cs
+using System.Numerics;
+
+namespace AcDream.Plugin.Abstractions;
+
+public readonly record struct WorldEntitySnapshot(
+    uint Id,
+    uint SourceId,
+    Vector3 Position,
+    Quaternion Rotation);