style: apply rustfmt across bench and core crates

Pure formatting pass - no semantic changes. Mostly long-line reflow,
import statement reordering (alphabetical), and trailing-newline fixes
that had been accumulating in the working tree.

Affected files:
* crates/bench/src/{data_gen,main,mmap_reader,pread_reader,report,runner}.rs
* crates/bench/src/suites/{concurrent,growth,jump,memory,render,rotation,startup}.rs
* crates/core/src/io/{file_reader,index_cache,read_cache}.rs
* crates/core/src/{types.rs, watcher/file_watcher.rs}

crates/core/src/io/read_cache.rs

@@ -492,4 +492,4 @@ mod tests {
         let err = cache.get(&file, u64::MAX, 1).unwrap_err();
         assert_eq!(err.kind(), io::ErrorKind::InvalidInput);
     }
-}
+}