Two behavior-preserving cleanups to the io_uring local read backend that
accumulated as later work layered onto it:
- Reunite the `UringBackend` struct doc comment. The backlog#1145 fd-cache
constants were inserted into the middle of the struct's doc block, so its
opening sentences were orphaned onto `ENV_RUSTFS_IO_URING_FD_CACHE` and the
struct itself was documented by a sentence fragment starting mid-clause with
"through rustfs-uring's...". Move the opening back onto the struct and give
the constant its own one-line doc.
- Fold the fd-cache handle and its lookup key into a single
`Option<(&FdCache, FdKey)>` in `pread_uring`, so the get and the
insert_if_fresh sites stop re-deriving `self.fd_cache.as_ref()` and
re-matching presence. Semantics are identical, including the backlog#1176
generation guard (`gen_at_open` snapshot before open, insert only when the
generation is unchanged).
No functional change. The borrow/move pattern was validated against a
host-compilable reduction (the cfg(linux) path cannot be built from macOS);
CI compiles the Linux backend.
Co-authored-by: heihutu <[email protected]>
