fix: update drifted source citations to match current upstream

Verified all 17 file:line citations against elixir-lang/elixir HEAD.
Fixed 10 citations where line numbers had shifted due to upstream changes:

- patterns/genserver.md: agent.ex:246 → 279 (start_link spec)
- patterns/process-design.md: task.ex:282 → 327 (child_spec)
- smells/anti-patterns.md: registry_test.exs:28 → 29, gen_server_test.exs:166 → 164,
  test_helper.exs:98 → 99
- smells/common-mistakes.md: registry_test.exs:28 → 29, callbacks.ex:423 → 433,
  task_test.exs:297,305,315,330 → 300,308,316,327,
  supervisor_test.exs:278 → 289, callbacks.ex:277 → 520

patterns/process-design.md

@@ -457,7 +457,7 @@ Supervisor.init(children,
 **Code example from source:**
 ```elixir
 # Task defaults to :temporary — intentional one-shot work
-# (from task.ex:282)
+# (from task.ex:327)
 def child_spec(arg) do
   %{
     id: Task,