1. HIGH: LinkedWriteFsync now uses SubmitLinkRequests (IOSQE_IO_LINK)
instead of SubmitRequests, ensuring write+fdatasync execute as a
linked chain in the kernel. Falls back to sequential on error.
2. HIGH: PreadBatch/PwriteBatch chunk ops by ring capacity to prevent
"too many requests" rejection when dirty map exceeds ring size (256).
3. MED: CloseBatchIO() added to Flusher, called in BlockVol.Close()
after final flush to release io_uring ring / kernel resources.
4. MED: Sync parity — both standard and io_uring paths now use
fdatasync (via platform-specific fdatasync_linux.go / fdatasync_other.go).
Standard path previously used fsync; now matches io_uring semantics.
On non-Linux, fdatasync falls back to fsync (only option available).
10 batchio tests, all blockvol tests pass.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
New package batchio/ with BatchIO interface (PreadBatch, PwriteBatch,
Fsync, LinkedWriteFsync) and standard sequential implementation.
Flusher refactored to use BatchIO: WAL header reads, WAL entry reads,
and extent writes are now batched through the interface. With the
default NewStandard() backend, behavior is identical to before.
UseIOUring config field added for future io_uring opt-in (Linux 5.6+).
9 interface tests, all existing blockvol tests pass unchanged.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
