…nload when gap exceeds limit (#21489)

Tightens the EngineBlockDownloader's behaviour when a NewPayload arrives
for a block whose parent is more than \`MaxReorgDepth\` blocks away from
the local head — the scenario behind #21311 / #20419.

- **\`execution/p2p/bbd.go\`** — \`downloadInitialHeader\`'s early-exit
check
  was one-sided (\`currentHead > headerNum\`), so when the EL was behind
  the new payload the check was skipped and the download wasted a
  \`loadPeers\` + 500-header batch fetch per slot before failing on the
  post-batch \`chainLen > limit\` check. Replaced with a symmetric
  \`AbsoluteDifference\` so the check fires in both directions.
-
**\`execution/engineapi/engine_block_downloader/block_downloader.go\`**
—
  added a short-circuit at the top of \`downloadBlocks\` for
\`NewPayloadTrigger\`. When \`abs(currentHead - parent) >
MaxReorgDepth\`
  we return \`ErrChainLengthExceedsLimit\` without spawning the bbd
  goroutine.
- \`download()\` now logs the chain-length-exceeded case at INFO rather
  than WARN, since per @taratorio's triage on #21311 it's expected
  behaviour, not an error — the EL relies on a later FCU (unbounded
  download) or staged sync to close the gap.

The underlying "Prysm doesn't always send a follow-up FCU" issue is
unchanged; that part remains tracked under #20419.

- [x] New unit tests in \`execution/p2p/bbd_test.go\` cover the
symmetric
  fail-fast check in both directions (EL behind / EL ahead).
- [x] New \`TestNewPayloadGapExceedsLimit\` table test in

\`execution/engineapi/engine_block_downloader/block_downloader_test.go\`
  covers the boundary conditions of the predicate.
- [x] \`make lint && make erigon integration\` clean.
- [ ] CI: \`sync-with-externalcl\` no longer log-spams the WARN line per
slot.

Closes #21309 (duplicate of #21311). Partial fix toward #21311 / #20419
— full resolution of those issues still needs the FCU side handled.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
(cherry picked from commit bc12fe0)