A multi-object delete spans many keys that route to different owner filers. The
gateway groups keys by owner and sends one batch per owner; the filer applies
each transaction under its own per-path lock, independent of the others.
A failed transaction (precondition or mutation error) is reported in its own
response without aborting the rest, matching S3 multi-object semantics where
each key succeeds or fails on its own. There is no cross-key atomicity, which S3
batch delete does not require.
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