fix(mobility_features): distanceTraveled was always 0

[Zeroupper]

_findMoves selected the path between two stops as previous <= t <= previous instead of previous <= t <= current, so every Move had an empty path and distanceTraveled was 0 for every MobilityContext (visible in the committed test/testdata/moves.json - every line has "distance":0.0).

• window now spans previous -> current
• when no samples are kept for the move (process restarted, _samples is RAM-only), fall back to Move.fromStops straight-line distance rather than 0
• _samples cleared with the rest of the day's state

Test: the existing 'path between locations' case now asserts distance > 1 km once a move exists (1.7 km, then 3.2 km); it was silently 0 before.

Bumps to 6.2.1.