docs: update all documentation for Phase 2 audio module (#4)

- README: add audio to hero text, comparison table, features list; add Audio
  Playback section with MorsePlayer, presets, WAV export, callbacks examples;
  update tree shaking to show /audio sub-path; mark Phase 2 complete in roadmap
- API.md: add full audio API reference — MorsePlayer class, constructor options,
  play/pause/stop/resume/dispose, sound presets table, WAV export functions
  (toWav, toWavBlob, toWavUrl, downloadWav), scheduler (buildSchedule,
  scheduleDuration), all audio types
- CHANGELOG: add v0.2.0 entry with audio features
- CONTRIBUTING: update bundle size targets (audio ≤ 6KB, full ≤ 12KB); add
  Adding Audio Features contribution guide

Co-authored-by: Vaibhav Rajput <vaibhav@Vaibhavs-MacBook-Pro.local>

Author: vaibhav1312