Add iouring_linux.go (build-tagged linux && !no_iouring) using
iceber/iouring-go for batched pread/pwrite/fdatasync. Includes
linked write+fsync chain for group commit optimization.
iouring_other.go provides silent fallback to standard on non-Linux.
blockvol.go wires UseIOUring config flag through to flusher BatchIO.
NewIOUring gracefully falls back if kernel lacks io_uring support.
10 batchio tests, all blockvol tests pass unchanged.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>