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/Tests/EditMode.meta

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Assets/_Project/Tests/EditMode/HeartbeatSystemTests.cs

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+using NUnit.Framework;
+using Unity.Entities;
+using ProjectM.Simulation;
+
+namespace ProjectM.Tests
+{
+    /// <summary>
+    /// Setup smoke test: proves the DOTS toolchain compiles, source-generates, and ticks.
+    /// Boots a bare ECS world, registers <see cref="HeartbeatSystem"/> in the
+    /// SimulationSystemGroup, and asserts the component advances once per group update.
+    /// (Netcode client/server world boot is covered separately by the §3 Play Mode check.)
+    /// </summary>
+    public class HeartbeatSystemTests
+    {
+        [Test]
+        public void Heartbeat_Advances_Once_Per_SimulationGroup_Tick()
+        {
+            using var world = new World("HeartbeatTestWorld");
+            var simulationGroup = world.GetOrCreateSystemManaged<SimulationSystemGroup>();
+            var heartbeatSystem = world.GetOrCreateSystem<HeartbeatSystem>();
+            simulationGroup.AddSystemToUpdateList(heartbeatSystem);
+            simulationGroup.SortSystems();
+
+            var entityManager = world.EntityManager;
+            var entity = entityManager.CreateEntity(typeof(Heartbeat));
+
+            Assert.AreEqual(0, entityManager.GetComponentData<Heartbeat>(entity).Tick,
+                "Heartbeat should start at 0.");
+
+            const int ticks = 8;
+            for (int i = 0; i < ticks; i++)
+            {
+                simulationGroup.Update();
+            }
+
+            Assert.AreEqual(ticks, entityManager.GetComponentData<Heartbeat>(entity).Tick,
+                "Heartbeat.Tick should advance exactly once per SimulationSystemGroup update.");
+        }
+    }
+}

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Assets/_Project/Tests/EditMode/ProjectM.Tests.EditMode.asmdef

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+{
+    "name": "ProjectM.Tests.EditMode",
+    "rootNamespace": "ProjectM.Tests",
+    "references": [
+        "ProjectM.Simulation",
+        "Unity.Entities",
+        "Unity.Collections",
+        "Unity.Mathematics",
+        "UnityEngine.TestRunner",
+        "UnityEditor.TestRunner"
+    ],
+    "includePlatforms": [
+        "Editor"
+    ],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": true,
+    "precompiledReferences": [
+        "nunit.framework.dll"
+    ],
+    "autoReferenced": false,
+    "defineConstraints": [
+        "UNITY_INCLUDE_TESTS"
+    ],
+    "versionDefines": [],
+    "noEngineReferences": false
+}

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