Set up DOTS + Netcode for Entities foundation

One-time stack setup per Docs/dots-setup-task.md (Unity 6.4.7 / 6000.4.7f1).
Packages: entities 6.4.0, entities.graphics 6.4.0, netcode 1.13.2, physics 1.4.6.

- Assets/_Project asmdef split: ProjectM.Simulation/Client/Server/Authoring (root ns ProjectM)
- GameBootstrap : ClientServerBootstrap; verified separate client + server worlds in Play Mode
- Gameplay subscene wired into SampleScene as a baking target
- Heartbeat component + Burst ISystem; EditMode smoke test green (1/1)
- In-repo Obsidian vault (Docs/Vault) incl. DR-001 (plain-Entities test over internal NetCodeTestWorld)
- Portable .mcp.json (basic-memory + serena via ${CLAUDE_PROJECT_DIR}); CLAUDE.md conventions
- .gitignore for DOTS baking cache + machine-local config

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Assets/_Project/Scripts/Simulation/GameBootstrap.cs

@@ -0,0 +1,24 @@
+using Unity.Entities;
+using Unity.NetCode;
+using UnityEngine.Scripting;
+
+namespace ProjectM.Simulation
+{
+    /// <summary>
+    /// Custom Netcode for Entities bootstrap. Subclassing <see cref="ClientServerBootstrap"/>
+    /// gives an explicit hook to customize world creation, tick rate, and auto-connect.
+    /// For now it reproduces the default behavior: create separate client and server worlds
+    /// based on the Multiplayer PlayMode Tools settings, without auto-connecting.
+    /// </summary>
+    [Preserve]
+    public class GameBootstrap : ClientServerBootstrap
+    {
+        public override bool Initialize(string defaultWorldName)
+        {
+            // 0 = do not auto-connect; worlds are still created. Set a port later to auto-connect.
+            AutoConnectPort = 0;
+            CreateDefaultClientServerWorlds();
+            return true;
+        }
+    }
+}