Extract _haversine_m from the inline block in _gps_speed_kmh, add
_spatial_downsample (keep one sample per 10 m traveled, GPS haversine
primary / speed×Δt fallback, indoor activities unchanged), and wire it
into build_timeseries() after the 1 s dedup loop.
Add --downsample-timeseries migration flag to bincio render that applies
the same downsampling to existing stored timeseries files without
re-extracting from original FIT/GPX files.
Davide Scaini
84eff1f3b0