Add on_write_buffer_drained

style.md

@@ -522,6 +522,21 @@ TEST_CASE("Server accepts connections") {
 // TEST_CASE("Server creates epoll instance") { /* implementation detail */ }
 ```
 
+### What NOT to Test
+
+**Avoid testing language features and plumbing:**
+- Don't test that virtual functions dispatch correctly
+- Don't test that standard library types work (unique_ptr, containers, etc.)
+- Don't test basic constructor/destructor calls
+
+**Test business logic instead:**
+- When does your code call hooks/callbacks and why?
+- What state transitions trigger behavior changes?
+- How does your code handle error conditions?
+- What promises does your API make to users?
+
+**Ask: "Am I testing the C++ compiler or my application logic?"**
+
 ### Test Synchronization (Authoritative Rules)
 - **ABSOLUTELY NEVER use timeouts** (`sleep_for`, `wait_for`, etc.)
 - **Deterministic synchronization only:**