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eth/protocols/snap: check pivot ancestry against the header chain - #35930
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Resolves #35922
When the pivot moves, snap/2 decides whether it can roll its state forward with BALs or has to start over because the old pivot was reorged out. It decided by looking up the canonical hash at the old pivot's height and treating a missing entry as a reorg. During snap sync that entry is usually missing even when nothing was reorged: the pivot block is only written to disk once its state is done, and the block download can trail the pivot by hours on a fresh sync. So a normal pivot move, a restart, or a lagging block download wiped all progress.
This PR checks the header chain instead. It follows parent hashes from the new pivot back to the old one's height, using the skeleton, which always has these headers. If it reaches the old pivot, we roll forward; otherwise we start over. The tests now keep their headers only in the skeleton, like a real sync, and fail with the old check.
TestPivotMovementwas passing on master by wiping and redownloading.