feat: add smart yank and footer flash indicator (v0.3.0)

- Add gy keymap (visual mode, terminal buffer) that strips PTY hard-wrap
  newlines before yanking; preserves genuine multi-line selections
- Add footer.yank_icon flash after clean yank; falls back to "Y" without
  Nerd Font (set to false to disable)
- Fix scroll lag: debounce CursorMoved updates and cache footer text to
  skip redundant nvim_win_set_config calls
- Fix horizontal scroll artifact: set wrap=true after termopen() so
  TermOpen autocmds cannot override it

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Release v0.2.0 - Customizable mode icons

Release v0.1.0 - Initial stable release