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/HeartbeatSystem.cs

@@ -0,0 +1,23 @@
+using Unity.Burst;
+using Unity.Entities;
+
+namespace ProjectM.Simulation
+{
+    /// <summary>
+    /// Smoke-test system: increments every <see cref="Heartbeat"/> once per tick.
+    /// Default (no <c>[WorldSystemFilter]</c>) so it runs in the SimulationSystemGroup of
+    /// every world. Burst-compiled and unmanaged (<see cref="ISystem"/>) per DOTS convention.
+    /// </summary>
+    [BurstCompile]
+    public partial struct HeartbeatSystem : ISystem
+    {
+        [BurstCompile]
+        public void OnUpdate(ref SystemState state)
+        {
+            foreach (var heartbeat in SystemAPI.Query<RefRW<Heartbeat>>())
+            {
+                heartbeat.ValueRW.Tick++;
+            }
+        }
+    }
+}