`with_events()` and `with_atomic_writes()` both install a writer, last call wins.
Appending them in the wrong order in the host would swap the transactional writer
back out for the best-effort one — silently reverting the write-path invariant AND
dropping the per-group quota enforcement that lives in it, with nothing in the type
system to catch it.
The services now carry an `atomic` flag: once `with_atomic_writes` has run,
`with_events` warns and no-ops instead of clobbering it. Also enforce the group
byte ceiling in `BestEffortGroupFilesWriter::update`, so the non-transactional
writer can't be used as an unmetered bypass and a unit test can catch its removal.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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