v2.0.0

A major release that overhauls the chord-generation pipeline and
introduces interactive practice. Highlights:

• Train mode — a Textual TUI backed by the FSRS spaced-repetition
  algorithm, with Anki-style daily quotas, per-word speed grading,
  leech detection, and an ASCII keyboard view that highlights chord
  keys.
• Drill mode — a read-only speed-drill TUI for words you've
  already graduated, with timer or word-count sessions and live WPM.
• Vocabulary pipeline — on-demand SUBTLEX downloads at setup
  time, with explicit frequency (Zipf) and category columns;
  reserve a chord by leaving its frequency cell empty.
• Optimal chord assignment — replaces the old greedy + 2-swap
  passes with a sparse minimum-cost bipartite matcher, plus
  alt-coverage filtering and optional frequency tiers.
• Redesigned alt generator — category/inflector registry instead
  of hard-coded UD POS tags, fully configurable from config.yaml.

See the changelog for more details