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

smells/anti-patterns.md

@@ -84,7 +84,7 @@ end)
 
 **What they avoid:** Tests that depend on or modify global state without cleanup.
 
-**Source evidence:** `lib/mix/test/test_helper.exs:98-113` — MixTest.Case restores ALL global state in `on_exit`:
+**Source evidence:** `lib/mix/test/test_helper.exs:99-115` — MixTest.Case restores ALL global state in `on_exit`:
 - `Mix.env(:dev)`, `Mix.target(:host)`, `Mix.Task.clear()`, `Mix.Shell.Process.flush()`
 - Unloads all applications that were loaded during the test
 
@@ -587,12 +587,12 @@ end
 
 **What they avoid:** ETS tables, registered names, or application env used across tests without isolation.
 
-**Source evidence:** `lib/elixir/test/elixir/registry_test.exs:28-31` — Each test gets a uniquely-named Registry:
+**Source evidence:** `lib/elixir/test/elixir/registry_test.exs:29-32` — Each test gets a uniquely-named Registry:
 ```elixir
 name = :"#{config.test}_#{partitions}_#{inspect(keys)}"
 ```
 
-`lib/elixir/test/elixir/gen_server_test.exs:166` — Uses `%{test: name}` for unique process registration.
+`lib/elixir/test/elixir/gen_server_test.exs:164` — Uses `%{test: name}` for unique process registration.
 
 **Why it's bad:** Tests that share state can't run concurrently. They're order-dependent and fragile.