fix(eth/downloader): don't mistake a lagging header head for a stalling peer

[gzliudan]

Proposed changes

Importing the post-pivot blocks moves the header head back to the block being inserted, so CurrentHeader can trail the headers the peer already delivered. When processHeaders reached the terminating batch inside that window, the fast sync TD check saw a head below the promised total difficulty and returned errStallingPeer, which rolled the whole chain back and dropped the only peer.

Track the highest header written to the light chain in this cycle and use it when it is ahead of CurrentHeader. Attack detection is unchanged: a peer that bails out of delivering the post-pivot headers never advances it.

This is what made TestFastSyncDisabling flaky under load: once the peer was dropped, BestPeer stayed nil and the sync loop could never recover.

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Impacted Components

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• Consensus
• Account
• Network
• Geth
• Smart Contract
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Checklist

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Tested on a private network from the genesis block and monitored the chain operating correctly for multiple epochs.
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A

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[Copilot]

Pull request overview

Fixes false errStallingPeer detection during fast sync when block imports temporarily move the current header backward.

Changes:

• Tracks the highest successfully inserted header for the current sync cycle.
• Uses that header for the terminating TD check when CurrentHeader lags.
• Adds regression tests across supported protocol versions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
eth/downloader/downloader.go	Corrects the fast/light sync stalling-peer check.
eth/downloader/downloader_test.go	Simulates header-head lag and verifies successful fast sync.

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