check formatting in ci and add a status badge

[Linskii]

the repository had drifted out of ruff format for two months without anyone
noticing, because ci ran tests and lint but never checked formatting. the
existing format task writes files, so it cannot gate; a format-check task
is added alongside it.

• ruff format . applied once across the repository, as its own commit. the
  change is provably cosmetic: the ast and the full comment sequence of all four
  touched files are identical before and after.
• ci gains a Format step, and lint now runs before the tests so a style failure
  reports in seconds rather than after the suite.
• the workflow gains a job timeout, so a deadlock in the multiprocessing tests
  cannot burn six hours, plus a concurrency group that cancels superseded pull
  request runs and a read-only token scope.
• a ci status badge at the top of the readme.