docs: add info about deadlocks in tests to usage rules

usage-rules.md

@@ -1131,3 +1131,53 @@ When testing resources:
 - Use `authorize?: false` in tests where authorization is not the focus
 - Write generators using `Ash.Generator`
 - Prefer to use raising versions of functions whenever possible, as opposed to pattern matching
+
+### Preventing Deadlocks in Concurrent Tests
+
+When running tests concurrently, using fixed values for identity attributes can cause deadlock errors. Multiple tests attempting to create records with the same unique values will conflict.
+
+#### Use Globally Unique Values
+
+Always use globally unique values for identity attributes in tests:
+
+```elixir
+# BAD - Can cause deadlocks in concurrent tests
+%{email: "test@example.com", username: "testuser"}
+
+# GOOD - Use globally unique values
+%{
+  email: "test-#{System.unique_integer([:positive])}@example.com",
+  username: "user_#{System.unique_integer([:positive])}",
+  slug: "post-#{System.unique_integer([:positive])}"
+}
+```
+
+#### Creating Reusable Test Generators
+
+For better organization, create a generator module:
+
+```elixir
+defmodule MyApp.TestGenerators do
+  use Ash.Generator
+
+  def user(opts \\ []) do
+    changeset_generator(
+      User,
+      :create,
+      defaults: [
+        email: "user-#{System.unique_integer([:positive])}@example.com",
+        username: "user_#{System.unique_integer([:positive])}"
+      ],
+      overrides: opts
+    )
+  end
+end
+
+# In your tests
+test "concurrent user creation" do
+  users = MyApp.TestGenerators.generate_many(user(), 10)
+  # Each user has unique identity attributes
+end
+```
+
+This applies to ANY field used in identity constraints, not just primary keys. Using globally unique values prevents frustrating intermittent test failures in CI environments.