First DOM-capable vitest lane for the repo: default env stays node (pure lib helpers), a test opts
into jsdom per-file with a // @vitest-environment jsdom docblock; plugin-react wired for JSX/hook tests.
Adds jsdom + @testing-library/react devDeps and 3 back-ladder tests (LIFO peel, stacks over
useBackToClose, inactive layer is transparent) driven by direct popstate dispatch — deterministic,
no dependency on jsdom's async back(). Closes the deferred R2 component-test gap (useBackToExit).