feat: count equivocator weight in gloas is_head_weak

[rahulbarmann]

Motivation

Gloas updates is_head_weak to also count weight from validators that are in head-slot committees AND in store.equivocating_indices (spec). Without it, head weight can decrease as equivocations land, breaking monotonicity.

Description

getProposerHead takes an optional equivocatingCommitteeWeightIncrements (default 0) added to headNode.weight before the threshold check. The chain layer computes it via a callback on updateAndGetHead since fork-choice has no committee access. New IForkChoice.getEquivocatingIndices() getter. Fork-gated to Gloas.

Scope kept to getProposerHead; shouldOverrideForkChoiceUpdate and should_apply_proposer_boost don't call is_head_weak either, but that's a pre-existing gap separate from this issue left for follow-up.

Tests cover the threshold boundary cases.

Closes #9232

AI Assistance Disclosure

• External Contributors: I have read the contributor guidelines and disclosed my usage of AI below.

Used Claude Code to draft the implementation. Reviewed every line and made the design calls myself.

[gemini-code-assist]

Code Review

This pull request implements the Gloas fork-choice modification, which accounts for the weight of equivocating validators in head-slot committees when determining if a head block is 'weak' and eligible for re-org. It introduces a mechanism to compute and pass this weight increment from the chain layer to the fork-choice layer, updates the relevant interfaces, and adds comprehensive unit tests to verify the new logic. I have no feedback to provide as there were no review comments.