The KawaiiSpinner animation would occasionally spam dozens of duplicate
lines instead of overwriting in-place with \r. This happened because
prompt_toolkit's StdoutProxy processes each flush() as a separate
run_in_terminal() call — when the write thread is slow (busy event loop
during long tool executions), each \r frame gets its own call, and the
terminal layout save/restore between calls breaks the \r overwrite
semantics.
Fix: rate-limit flush() calls to at most every 0.4s. Between flushes,
\r-frame writes accumulate in StdoutProxy's buffer. When flushed, they
concatenate into one string (e.g. \r frame1 \r frame2 \r frame3) and
are written in a single run_in_terminal() call where \r works correctly.
The spinner still animates (flush ~2.5x/sec) but each flush batches
~3 frames, guaranteeing the \r collapse always works. Most visible
with execute_code and terminal tools (3+ second executions).
e4adb67ed8