Stream restore plan selection across levels

[benbjohnson]

Summary

• refactor CalcRestorePlan to stream iterators per level, picking the next file with the longest contiguous TXID range instead of collecting all level files first
• add level cursors that concurrently advance through iterators, allowing gaps at higher levels to be filled by longer ranges from lower levels
• expand test coverage to ensure the new planner prefers longer candidates and can recover from gaps using lower levels

Testing

• Not run (not requested)

[benbjohnson]

I tested this against 350+ databases internally and they all pass with PRAGMA integrity_check. This PR also include a fuzz test which was run for a while too.