Replace UseIOUring bool with IOBackend IOBackendMode (tri-state):
- "standard" (default): sequential pread/pwrite/fdatasync
- "auto": try io_uring, fall back to standard with warning log
- "io_uring": require io_uring, fail startup if unavailable
NewIOUring now returns ErrIOUringUnavailable instead of silently
falling back — callers decide whether to fail or fall back based
on the requested mode. All mode transitions are logged:
io backend: requested=auto selected=standard reason=...
io backend: requested=io_uring selected=io_uring
CLI: --io-backend=standard|auto|io_uring added to iscsi-target.
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>
