Fix code quality: trailing whitespace, unused imports, clippy warnings

- Fix trailing whitespace in kex.rs and s3.rs
- Add missing KexProposal import in kex_complete.rs
- Auto-fix clippy warnings across all crates
- All 153 tests pass

markbase-nfs/src/lib.rs

@@ -1,4 +1,4 @@
 pub mod nfs;
 
-pub use nfs::markbase_fs::MarkBaseFS;
 pub use nfs::backend::MarkBaseNFSBackend;
+pub use nfs::markbase_fs::MarkBaseFS;