feat(s1): MIDI import/export with bar-aligned phrases Adds griff-core::midi with import/export/summarise over midly 0.5. Tracks are reconstructed into Bar-grouped Phrase values; tempo and time-signature metadata are threaded through from global MIDI events. CLI gains three subcommands: import (one-line summary per track), inspect (bar-by-bar dump), export (roundtrip write). All paths, error types and the main entry-point satisfy the strict clippy policy (no process::exit, no &PathBuf args, no absolute paths in bodies). 40 tests pass; cargo fmt and cargo clippy --all-targets -D warnings are both clean. - #3