patterns(testing): add async test filtering pattern (#21)

When testing telemetry, pub/sub, or any broadcast mechanism with
async: true, events from concurrent tests can leak into mailbox.

Fix: Pin-match on unique identifier to filter.

Two-part pattern:
1. Filter at source — only forward events matching test's identifier
2. Pin in assertion — ^variable rejects mismatches that slip through

Applies to telemetry handlers, PubSub, GenStage/Broadway consumers,
any shared message bus.

Triggered by PR #710 flaky test fix in gargoyle.

patterns/testing.md

@@ -1772,3 +1772,76 @@ end
 - If you want to make test dependencies on setup context explicit → [Context Pattern Matching](#20-context-pattern-matching-in-test-signatures)
 
 <!-- PATTERN_COMPLETE -->
+
+## 21. Filtering Events in Async Tests
+
+**Source:** Gargoyle PR #710 (flaky telemetry test fix)
+
+**What it does:** Pin-matches on a unique identifier to filter events from concurrent tests.
+
+**Why:** When testing telemetry, pub/sub, or any broadcast mechanism with `async: true`, events from other tests can leak into your mailbox. Without filtering, tests pass in isolation but fail randomly when run in parallel.
+
+**Pattern:**
+
+Wrong — receives events from other tests:
+```elixir
+test "emits telemetry on create", %{user: user} do
+  :telemetry.attach("test", [:user, :created], &send_to_test/4, self())
+  create_post(user)
+  assert_receive {:telemetry, %{user_id: uid}}
+  assert uid == user.id  # Might match event from another test!
+end
+```
+
+Right — pin filters to only your test's events:
+```elixir
+test "emits telemetry on create", %{user: user} do
+  test_pid = self()
+  expected_uid = user.id
+
+  :telemetry.attach("test-#{inspect(test_pid)}", [:user, :created], fn _, _, meta, pid ->
+    if meta.user_id == expected_uid, do: send(pid, {:telemetry, meta})
+  end, test_pid)
+
+  create_post(user)
+  assert_receive {:telemetry, %{user_id: ^expected_uid}}
+end
+```
+
+### Key Insight
+
+The fix happens in two places:
+
+1. **Filter at the source** — only send messages that match your test's unique identifier
+2. **Pin in the assertion** — use `^variable` to reject mismatches that slip through
+
+### When to Use
+
+**Triggers:**
+- Telemetry handlers (filter by user_id, request_id, or test pid)
+- Phoenix.PubSub subscriptions (use unique topic per test)
+- GenStage/Broadway consumers (tag events with test pid)
+- Any shared message bus in async tests
+- Tests pass alone but fail with `--seed` or in CI
+
+### When NOT to Use
+
+**Don't use this when:**
+- Tests run with `async: false` (no concurrent tests to leak events)
+- You control the event source and can make it test-aware by design
+- The event contains a natural unique key you already have (just pin on it)
+
+**Over-application example:**
+```elixir
+# Overkill when you already have a unique key
+test "user update", %{user: user} do
+  # user.id is already unique — just pin it directly
+  assert_receive {:updated, %{id: ^user.id}}
+end
+```
+
+### Decision Tree Addition
+
+- If async tests randomly fail due to events from other tests → [Filtering Events in Async Tests](#21-filtering-events-in-async-tests)
+
+<!-- PATTERN_COMPLETE -->