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/ecstore/src/set_disk/ops/heal.rs

@@ -222,7 +222,7 @@ impl SetDisks {
                         }
 
                         if !latest_meta.deleted && !latest_meta.is_remote() {
-                            for (_, part_errs) in data_errs_by_part.iter() {
+                            for part_errs in data_errs_by_part.values() {
                                 if count_part_not_success(part_errs) > latest_meta.erasure.parity_blocks {
                                     cannot_heal = true;
                                     break;