feat: initialize OECS project structure

Initializes the OECS repository with the core foundation, including:

- Entity management with 24-bit IDs and 8-bit versioning
- Sparse set-based component storage
- Entity allocation and recycling logic
- Core World API for entity and component lifecycle
- Project scaffolding for source and unit tests
- Design documentation and architecture decision records (ADRs)

.gitignore

@@ -0,0 +1,33 @@
+# Build results
+[Dd]ebug/
+[Rr]elease/
+x64/
+x86/
+[Bb]in/
+[Oo]bj/
+*.user
+*.suo
+*.cache
+
+# NuGet
+*.nupkg
+**/[Pp]ackages/*
+!**/[Pp]ackages/build/
+*.nuget.props
+*.nuget.targets
+
+# IDE
+.vs/
+.vscode/
+.idea/
+*.swp
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Test results
+TestResults/
+*.trx
+*.coverage