block: Add AlignedFile

Provide a single home for O_DIRECT alignment and RMW behavior behind an
std::os::unix::fs::FileExt implementation built on AlignedBuffer.

Unaligned requests are bounced through an AlignedBuffer (applying RMW
for writes) and aligned requests pass straight through to the inner
File.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Rob Bradford <rbradford@meta.com>

block/src/lib.rs

@@ -15,6 +15,7 @@ pub mod factory;
 mod io_impl;
 pub use io_impl::{async_io, fcntl, request};
 pub(crate) mod aligned_buffer;
+pub(crate) mod aligned_file;
 pub mod formats;
 mod sparse;
 use std::alloc::{Layout, alloc_zeroed};