Add "force concurrent execution" to style guide

style.md

@@ -417,6 +417,27 @@ TEST_CASE("Server accepts connections") {
   - `std::latch`, `std::barrier`, futures/promises
   - RAII guards and resource management
 
+### Multithreading Test Correctness
+- **Force concurrent execution** - Thread creation takes time, so work often completes sequentially before threads start
+- **Use std::latch to synchronize thread startup** - Ensures all threads begin racing simultaneously
+```cpp
+// BAD: Race likely over before threads start
+std::atomic<int> counter{0};
+for (int i = 0; i < 4; ++i) {
+  threads.emplace_back([&]() { counter++; }); // Probably sequential
+}
+
+// GOOD: Force threads to race simultaneously
+std::atomic<int> counter{0};
+std::latch start_latch{4};
+for (int i = 0; i < 4; ++i) {
+  threads.emplace_back([&]() {
+    start_latch.count_down_and_wait(); // All threads start together
+    counter++; // Now they actually race
+  });
+}
+```
+
 ---
 
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