_find_cheapest_window_in_pool() scans for contiguous available blocks
when scheduling multiple tasks. When a block-in-progress hit a
temporal gap (e.g. from price-level filtering removing intervals from
the middle of the search range, or missing API data), the scanner
jumped to i = j + 1 instead of i = j, silently skipping the interval
right after the gap as a valid — sometimes cheaper — window start.
Distinguish 'unavailable slot' (correctly skipped via j + 1) from
'temporal gap' (must retry at j, since that slot was never actually
tested as a window start).
Impact: find_cheapest_schedule can now find the true cheapest window
for a task when the search range contains time gaps, instead of
occasionally picking a more expensive window right after such a gap.
