The handler copies the store's bytes before hashing and sends that copy, so a
reused store buffer cannot change the answer. The directory store opens with
O_NONBLOCK so a FIFO named by a hash is refused by fstat instead of holding a
libuv thread. New tests: exact record-key compare and blank keys, empty-hash
prefix lookalikes, single store read, buffer reuse, FIFO (POSIX, uid-independent).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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