feat(curses): add configurable stripchart scale

[Komzpa]

Summary

This is a modern replacement for the old fixed-scale experiment in #227, following the shape discussed there.

The default stripchart behavior remains dynamic. A fixed scale is only used when requested with:

• --scale fast
• --scale average
• --scale slow
• --scale 5:15:25:35:45:55:101:200:400 style custom thresholds

The average preset uses the threshold sequence Roger suggested in #227. Custom scales require nine strictly increasing millisecond thresholds, matching the stripchart symbols before the final overflow marker.

Supersedes #227.

Validation

• make -j "$(nproc)"
• git diff --check
• sudo python3 ./test/cmdparse.py
• ./mtr --help
• ./mtr --report --scale average -c 1 127.0.0.1
• ./mtr --report --scale 5:15:25:35:45:55:101:200:400 -c 1 127.0.0.1
• ./mtr --report --scale 5:4:25:35:45:55:101:200:400 -c 1 127.0.0.1 rejects the non-increasing scale
• timeout 3s script -q -c 'TERM=xterm ./mtr --displaymode 1 --scale average 127.0.0.1' /tmp/mtr-scale.typescript showed Scale: .:5 ms ... c:400 ms >

[Komzpa]

Superseded by the updated #580, which now carries the stripchart scale option together with the related curses option plumbing so the merge stack no longer has an internal ui/mtr.c conflict.