CwProbe.step() appended one line per call with no size limit, rotation or
cleanup, and it runs on every build: CwDeepDecoder is the only ICwDecoder
implementation and writes infer_begin + infer_done every 1.5 s inference tick.
Measured against the actual line format that is ~170 KB/hour, ~4 MB/day of
unbounded growth in files/probe_cw.txt while CW audio is monitored, plus
synchronous disk I/O on every inference.
Truncate when the file exceeds 1 MiB instead of deleting, so the probe keeps
the most recent diagnostics (the reason it exists: the last lines show where a
flash-crash died). Simulated 10 h of continuous use: 2.7 MB written in total,
file stays bounded around ~640 KB; previously it would have kept all 2.7 MB
and grown without limit.