heh (HEx Helper) is a cross-platform terminal hex editor written in Rust. It provides a TUI for viewing and editing binary files in both hexadecimal and ASCII representations, with features like search (hex/ASCII), jump-to-byte, undo, clipboard integration, and configurable endianness. The architecture follows a clean separation of concerns: `app.rs` orchestrates state and event handling, `screen.rs` manages rendering via ratatui, `input.rs` dispatches keyboard/mouse events to pluggable `KeyHandler` implementations (editor, popups), and `buffer.rs` implements an asynchronous memory-mapped buffer for responsive editing of large files. The codebase demonstrates strong Rust practices, including use of traits for extensibility, iterator patterns for decoding, and a custom `OverlappingChunks` iterator for handling UTF-8 boundary issues. It is primarily an application but includes a library interface for embedding as a ratatui widget.