Update proposer boost calculation

[michaelsproul]

Issue Addressed

Closes:

• Implement updated update_proposer_boost_root in fork choice #8689

Proposed Changes

• Calculate the proposer index on the canonical chain (from canonical head) at slot and plumb it through to fork choice so it can be used to determine whether or not to apply the proposer boost. We use the proposer cache to handle state advances and avoid duplicate work.
• Update our FC tests to use block.message().proposer_index() (always pass), we are not attempting to test this feature in those tests. The EF tests use the correct canonical proposer idnex via on_block, except for invalid blocks which just auto-pass this check (these blocks get rejected by other checks in on_block anyway).

Additional Info

This change may help us pass more of the fork choice compliance tests:

• ef_tests: wire fork-choice compliance suites #9185

[mergify]

Some required checks have failed. Could you please take a look @michaelsproul? 🙏

[michaelsproul]

Working on the test failure now

[jimmygchen]

LGTM!

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-29 11:51 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking unstable (16132a3), #9221 and #9215 together #9224
• ✅ Merged — 2026-04-29 12:19 UTC · at 6841dc5ec59c72a59ce3e41a57cf8fab20eb7ad8 · squash

This pull request spent 28 minutes 46 seconds in the queue, including 27 minutes 23 seconds running CI.

Required conditions to merge

• #approved-reviews-by >= 1 [🛡 GitHub branch protection]
  • Update proposer boost calculation #9215
• #changes-requested-reviews-by = 0 [🛡 GitHub branch protection]
  • Update proposer boost calculation #9215
• check-success=local-testnet-success
• check-success=test-suite-success