chore(ecstore): remove test-only BitrotErrorType and pin wire-only disk variants (#6032)

BitrotErrorType (disk/error.rs) was constructed only by its own unit test: production bitrot mismatches never flow through it (they surface as DiskError::other strings). Delete the enum, its From<BitrotErrorType> for DiskError impl, the self-test, and the api facade re-export. The facade inventory doc does not name the type, so no doc change is needed.

DiskError::SourceStalled and DiskError::CrossDeviceLink are never constructed locally — they are reachable only through wire decoding and no current node sends them. Their decode arms stay per the cross-version compatibility constraint; each variant now carries a doc comment saying exactly that so the next dead-code sweep does not re-litigate them. Their consumer arms (heal classifier, batch processor) are left untouched — the values cannot appear, so removing the arms would be unobservable, and the heal classifier is pinned by the issue as do-not-touch.

Ref rustfs/backlog#1831 (PR4).

crates/ecstore/src/api/mod.rs

@@ -346,7 +346,7 @@ pub mod disk {
     }
 
     pub mod error {
-        pub use crate::disk::error::{BitrotErrorType, DiskError, Error, FileAccessDeniedWithContext, Result};
+        pub use crate::disk::error::{DiskError, Error, FileAccessDeniedWithContext, Result};
     }
 
     pub mod error_reduce {