KawaiiSpinner used a two-phase clear+redraw approach: first write
\r + spaces to blank the line, then \r + new frame. When running
inside prompt_toolkit's patch_stdout proxy, each phase could trigger
a separate repaint, causing visible flickering every 120ms.
Replace with a single \r\033[K (carriage return + ANSI erase-to-EOL)
write so the line is cleared and redrawn atomically.
f92875bc3e