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/common-mistakes.md

@@ -91,7 +91,7 @@ setup do
 end
 ```
 
-**Source:** `lib/ex_unit/lib/ex_unit/callbacks.ex:277-340` — `start_supervised` is designed specifically for this: guaranteed shutdown in reverse order, no leaked processes, no race conditions.
+**Source:** `lib/ex_unit/lib/ex_unit/callbacks.ex:520-568` — `start_supervised` is designed specifically for this: guaranteed shutdown in reverse order, no leaked processes, no race conditions.
 
 ### When to Apply This Rule
 
@@ -236,7 +236,7 @@ test "starts a server", %{test: test_name} do
 end
 ```
 
-**Source:** `lib/elixir/test/elixir/registry_test.exs:28` — `name = :"#{config.test}_#{partitions}_#{inspect(keys)}"` — always derives unique names from test context.
+**Source:** `lib/elixir/test/elixir/registry_test.exs:29` — `name = :"#{config.test}_#{partitions}_#{inspect(keys)}"` — always derives unique names from test context.
 
 ### When to Apply This Rule
 
@@ -474,7 +474,7 @@ describe "admin users - deletion" do
 end
 ```
 
-**Source:** `lib/ex_unit/lib/ex_unit/callbacks.ex:423-425` — `no_describe!` check prevents nesting.
+**Source:** `lib/ex_unit/lib/ex_unit/callbacks.ex:433-437` — `no_describe!` check prevents nesting.
 
 ### When to Apply This Rule
 
@@ -628,7 +628,7 @@ test "handles process crash" do
 end
 ```
 
-**Source:** `lib/elixir/test/elixir/task_test.exs:297,305,315,330` — Every test that expects a linked process to crash sets `:trap_exit` first.
+**Source:** `lib/elixir/test/elixir/task_test.exs:300,308,316,327` — Every test that expects a linked process to crash sets `:trap_exit` first.
 
 ### When to Apply This Rule
 
@@ -866,7 +866,7 @@ test "process stops" do
 end
 ```
 
-**Source:** `lib/elixir/test/elixir/supervisor_test.exs:278-285` — `assert_kill` helper always uses monitor + assert_receive, never `Process.alive?` polling.
+**Source:** `lib/elixir/test/elixir/supervisor_test.exs:289-293` — `assert_kill` helper always uses monitor + assert_receive, never `Process.alive?` polling.
 
 ### When to Apply This Rule