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/index_cache.rs

@@ -1,7 +1,7 @@
 use std::io::{Read as _, Write as _};
 use std::path::Path;
 
-use crate::io::cache_util::{cache_path, CACHE_VERSION};
+use crate::io::cache_util::{CACHE_VERSION, cache_path};
 use crate::io::line_index::LineIndex;
 
 pub struct IndexCache;
@@ -47,11 +47,7 @@ impl IndexCache {
     ///
     /// The file hash is derived from `data` (the same byte slice used to build the index),
     /// avoiding TOCTOU issues from re-reading the file from disk.
-    pub fn save_with_hash(
-        file_path: &Path,
-        index: &LineIndex,
-        data: &[u8],
-    ) -> std::io::Result<()> {
+    pub fn save_with_hash(file_path: &Path, index: &LineIndex, data: &[u8]) -> std::io::Result<()> {
         let dest = cache_path(file_path).ok_or_else(|| {
             std::io::Error::new(std::io::ErrorKind::NotFound, "cannot determine cache path")
         })?;
@@ -382,6 +378,10 @@ mod tests {
         }
 
         let final_data = std::fs::read(&dest).expect("dest file should exist");
-        assert_eq!(final_data.len(), 64 * 1024, "final file must be exactly one payload");
+        assert_eq!(
+            final_data.len(),
+            64 * 1024,
+            "final file must be exactly one payload"
+        );
     }
-}
+}