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-core/src/scan.rs

@@ -439,7 +439,7 @@ fn compute_hashes_parallel(
 
                 let mut p = processed.lock().unwrap();
                 *p += 1;
-                if *p % 100 == 0 {
+                if (*p).is_multiple_of(100) {
                     print!("\r      Hashed {}/{} files...", *p, total);
                     use std::io::Write;
                     std::io::stdout().flush().ok();