Review fold: build "\a" + sequence once and dispatch it to exactly one sink (app loop
when the Application runs, write_tty otherwise) instead of two branches with two writes
and two try/excepts. terminal_notify.notify() had a single caller (that fallback), so
write_tty is the public no-app entry and notify() is gone. The loop-side write keeps the
never-raises contract on a dead tty (EIO/closed file) the same way _pet_flush_kitty_frame
does, and _ring_bell skips entirely once _terminal_io_broken is set.
pty A/B re-run on this stack: 400 rings vs 90 frames, 0 aborted, 0 painted, 400/400 delivered.
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