build(toolchain): switch Rust channel to stable (#4775)

* Change Rust toolchain channel to stable

Signed-off-by: houseme <housemecn@gmail.com>

* style: apply clippy --fix and cargo fix lint suggestions

Run `cargo clippy --fix --all-targets --all-features` and
`cargo fix --lib --all-targets` across the workspace, then resolve the
remaining warnings by hand:

- collapse needless borrows in `format!` args, prefer `?` over explicit
  early returns, and use `.values()` / `.flatten()` iterator adapters
- rewrite the `Md5` scan loop via `manual_flatten` and re-indent the
  `select!` macro body (rustfmt skips macro interiors)
- annotate the intentional dead-code `Md5` inherent methods (constructed
  only by the test factory) with `#[allow(dead_code)]`

Behavior is unchanged.

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Signed-off-by: houseme <housemecn@gmail.com>
Co-authored-by: heihutu <heihutu@gmail.com>

crates/filemeta/src/filemeta.rs

@@ -43,7 +43,7 @@ use uuid::Uuid;
 use xxhash_rust::xxh64;
 
 // XL header specifies the format
-pub static XL_FILE_HEADER: [u8; 4] = [b'X', b'L', b'2', b' '];
+pub static XL_FILE_HEADER: [u8; 4] = *b"XL2 ";
 // pub static XL_FILE_VERSION_CURRENT: [u8; 4] = [0; 4];
 
 // Current version being written.