Require a 3.5% minimum slope on detected climbs, re-segmenting gentler stretches

A detected climb must now average at least 3.5% slope with re-segmentation:
gentle drags are split into their qualifying steeper parts instead of being
reported wholesale or dropped. Qualifying climbs keep their previous
boundaries exactly; only regions whose classic best climb averages below the
threshold get re-segmented (full-scan for the best qualifying sub-segment,
found=false when none exists).

QA: A/B across data/rides (362 rides) went from 137 to 133 climbs, 23 rides
changed; the 7 fully-dropped rides were audited to contain zero qualifying
sub-segments. Example-ride tests updated for the new behavior; the resolved
"minimum slope" TODO was removed.

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