Add thread safety documentation

design.md

@@ -56,6 +56,7 @@ Key features:
 - Proper alignment handling for all types
 - Move semantics for efficient transfers
 - Requires trivially destructible types only
+- Thread-safe per-connection usage via exclusive ownership model
 
 #### 2. **Commit Request Data Model** (`src/commit_request.hpp`)
 - **Format-agnostic data structure** for representing transactional commits
@@ -265,6 +266,7 @@ The modular design allows each component to be optimized independently while mai
 
 ### Important Implementation Details
 - **Server Creation**: Always use `Server::create()` factory method - direct construction is impossible
+- **Connection Creation**: Only the Server can create connections - no public constructor or factory method
 - **Connection Ownership**: Use unique_ptr semantics for safe ownership transfer between components
 - **Arena Allocator Pattern**: Always use `ArenaAllocator` for temporary allocations within request processing
 - **String View Usage**: Prefer `std::string_view` over `std::string` when pointing to arena-allocated memory
@@ -323,7 +325,9 @@ The modular design allows each component to be optimized independently while mai
 
 ## Common Patterns
 
-### Server Creation Pattern
+### Factory Method Patterns
+
+#### Server Creation
 ```cpp
 // Server must be created via factory method
 auto server = Server::create(config, handler);
@@ -333,6 +337,20 @@ auto server = Server::create(config, handler);
 // auto server = std::make_shared<Server>(config, handler);  // Compiler error
 ```
 
+#### Connection Creation (Server-Only)
+```cpp
+// Only Server can create connections (using private friend method)
+class Server {
+private:
+  auto conn = Connection::createForServer(addr, fd, id, handler, weak_from_this());
+};
+
+// No public way to create connections - all these fail:
+// auto conn = Connection::create(...);        // ERROR: no such method
+// Connection conn(addr, fd, id, handler, server);  // ERROR: private constructor
+// auto conn = std::make_unique<Connection>(...);   // ERROR: private constructor
+```
+
 ### ConnectionHandler Implementation Patterns
 
 #### Simple Synchronous Handler