Multi-resolution sequence builder reads from
[anchor + total - lookback, anchor + total), so anchor must be in
[max(0, lookback - total), snapshots.len() - (total + max_horizon)).
The previous gen_range(0..max_anchor) underflowed for default
3-scale (lookback=1510, total=32) at small anchors. Compute
min_anchor = lookback.saturating_sub(total) and sample
gen_range(min_anchor..max_anchor) with the ensure! upgraded to
check max_anchor > min_anchor.
Caught by implementer self-review of SDD-MR Task 3.
dcbc51f432