v2.2.0

v2.2.0 adds book mode (chordgen book PATH), renames train → learn, and includes several learn-mode refinements:

Book mode — Type through any .txt/.md/.epub at your own pace. Auto-resumes via content hash, highlights learned words in yellow, reveals chords on mistype, scrolls by line.
Learn mode — Two-phase chord reveal during initial learning, separate learning/relearn step counts, calendar-date overdue check (Anki-style), chord stays hidden during relearning, red flash no longer blocks input.
Drill mode — Arbitrary word lists (--words-file), per-keyboard PB leaderboard.
Chord assignment — frequency_exponent raised to 3.0, key replacement for missing layout keys.

See the changelog for more details