docs(relay): address the teardown ratchet's docstring to whoever it goes red on

The reader who decides this test's fate is not the reviewer — it is someone
hitting a red ratchet during an unrelated refactor, weighing whether to relax
the pattern. Say the trade to them directly: a ratchet that fails loudly on a
benign change beats one that passes silently on a harmful one, and the literal
match is deliberate because a refactor that obscures which teardown runs at
quit is exactly what should get a second look.

src/main/startup/desktop-relay-teardown-call-site-ratchet.test.ts

@@ -14,6 +14,10 @@ import { describe, expect, it } from 'vitest'
  * This test is the compile error that swap cannot produce. Both directions matter: quit must not
  * regain the re-armable fence, and sign-out/relaunch must not lose it — collapsing them either way
  * destroys the distinction the defect came from.
+ *
+ * If you are here because this went red on a refactor that looks harmless, that is the intended
+ * trade: a ratchet that fails loudly on a benign change beats one that passes silently on a
+ * harmful one. Re-read which teardown your call site now runs before relaxing the pattern.
  */
 const STARTUP_DIR = __dirname
 const QUIT_MODULE = 'main-process-quit.ts'