fix(sync): sync distant P2P data heads by range

[jgimeno]

Overview

Fix P2P data head verification so non-adjacent heads are accepted as provisional sync targets instead of triggering repeated bifurcation requests.

P2PData.Verify previously compared the trusted data hash with LastDataHash for every candidate. That linkage is only meaningful for adjacent heights: a distant head points to its immediate predecessor, not the node's current local head. As a result, distant heads soft-failed verification and go-header fetched intermediate heights repeatedly before starting normal range sync (258 GetByHeight calls when catching up from height 1 to 64 in the regression).

The verification now checks data-hash linkage only for adjacent heights. Non-adjacent heads can become provisional sync targets, after which the downloaded range is verified link by link. Adjacent candidates with an invalid LastDataHash are still rejected.

Validation

• go test ./... -count=1
• go test ./pkg/sync -run TestDataSyncerDistantHeadUsesRangeSync -count=50
• go test -race ./pkg/sync -run TestDataSyncerDistantHeadUsesRangeSync -count=1
• go test ./types -run TestP2PDataVerifyAdjacentHeads -count=50
• go vet ./types ./pkg/sync
• git diff --check

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