log crate facade for pluggable logging backends

The log crate provides logging macros (error!, warn!, info!, debug!, trace!) that are backend-agnostic. Libraries use log, and applications choose the backend (env_logger, simple_logger, or tracing). This decouples logging from the implementation. I use log in libraries and env_logger in binaries for quick setup. The RUST_LOG environment variable controls verbosity: RUST_LOG=debug cargo run. For structured logging, I prefer tracing, but log is simpler and ubiquitous. The facade pattern means changing the logging backend doesn't break libraries, which is why log is the de facto standard in the Rust ecosystem.