mailrs-delivery-executor 1.2.0

Group-commit delivery executor on top of mailrs-maildir 1.2's deliver_batch. Accumulates per-path delivery requests from concurrent async tasks (SMTP / LMTP / IMAP APPEND sessions) and flushes each path's batch via a single fsync, with optional pipelined in-flight flushes (max_concurrent_flushes=2 default) to hide disk wait behind batch collection. Caller awaits a oneshot::Receiver for per-message confirmation. Default tuning: max_batch=64, max_wait=10ms, max_concurrent_flushes=2.