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>

.gitignore

@@ -105,3 +105,17 @@ InitTestScene*.unity*
 
 # Auto-generated scenes by play mode tests
 /[Aa]ssets/[Ii]nit[Tt]est[Ss]cene*.unity*
+
+# DOTS / Entities subscene baking cache (regenerated by the bake pipeline)
+/[Aa]ssets/SceneDependencyCache/
+/[Aa]ssets/SceneDependencyCache.meta
+
+# macOS
+.DS_Store
+
+# Claude Code — share settings.json, keep local overrides machine-local
+.claude/settings.local.json
+
+# Obsidian — share vault config, ignore per-machine UI/workspace state
+[Dd]ocs/[Vv]ault/.obsidian/workspace.json
+[Dd]ocs/[Vv]ault/.obsidian/workspace-mobile.json