fix(core): store gap snapshot before writeHeadBlock to close crash window

[gzliudan]

Proposed changes

UpdateMasternodes (called via UpdateM1) was invoked after writeHeadBlock, leaving a window where the node head pointed to a gap block but no corresponding snapshot existed in the database. If the process was killed or restarted during the lengthy per-candidate EVM calls in UpdateM1, the snapshot would never be written. On the next boot the node would load the gap block as its head, then fail with:

Cannot find snapshot from last gap block err="leveldb: not found"

on every block in the following epoch, silently dropping out of consensus.

Fix: add updateM1ForBlock(block, statedb) which reads candidates and stakes directly from the committed state trie (same as downloader.generateSnapshot) without depending on bc.CurrentBlock() or bc.CurrentHeader(). Both the canonical-chain path (writeBlockWithState) and the reorg path (reorg) now call updateM1ForBlock with bc.StateAt(block.Root()) before writeHeadBlock, so the snapshot is durable before the head markers are persisted.

The original UpdateM1 is retained unchanged for external callers and tests.

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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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