- Reduce default overlay-final-timeout from 4s to 3s and validate
that non-zero timeout is at least 3s to prevent captions from
disappearing too quickly.
- Limit completed captions to six lines and keep each visible for
at least 3s, improving readability during fast speech.
- Update README to document new VAD behavior and changed defaults.
Move the executable entry point to `src/cmd/captioneer` and split audio, capture, config, caption, and model logic into separate packages under `src/`. Update README with new build/run commands and a package-overview section.