fix: audit fixes for 4 medium-severity bugs: index_cache tail hash, read_cache doc, mutex poison recovery, Remove event handling

crates/core/src/io/index_cache.rs

@@ -86,11 +86,12 @@ fn compute_file_hash(file_path: &Path) -> std::io::Result<u64> {
         let mut tf = std::io::BufReader::new(file);
         tf.read_exact(&mut tail)?;
     } else {
-        // File is small enough that head already covers everything;
-        // tail overlaps with head — just take the last tail_size bytes
-        let start = head.len().saturating_sub(tail_size);
-        tail = head[start..].to_vec();
-        tail.resize(tail_size, 0);
+        // File fits within head+tail window. Seek to read the real tail
+        // for correctness — approximating from head misses bytes beyond head_size.
+        let tail_start = file_size.saturating_sub(tail_size as u64);
+        let mut file = std::fs::File::open(file_path)?;
+        std::io::Seek::seek(&mut file, std::io::SeekFrom::Start(tail_start))?;
+        file.read_exact(&mut tail)?;
     }
 
     let mut hasher_state = xxhash_rust::xxh3::Xxh3::new();
@@ -242,4 +243,4 @@ mod tests {
 
         assert_ne!(h1, h2, "hash should change when content changes");
     }
-}
+}