fix(fork-choice): align proposer reorg helper with consensus spec

[VanshSahay]

Fixes two divergences between Lighthouse's get_proposer_head_info and the consensus spec:

1. Finalization age check now uses store.finalized_checkpoint.epoch instead of head_node.unrealized_finalized_checkpoint().epoch. The store's checkpoint is the ratified value; the head's unrealized checkpoint is optimistic and unreliable when considering a reorg.

2. FFG competitiveness no longer requires unrealized_finalized_checkpoint to match between parent and head — only unrealized_justified_checkpoint. This matches the spec's is_ffg_competitive which only compares justifications. Finalization follows from justification, so blocking on finalized checkpoints gives up useful reorgs for no safety gain.

3. Removed the now-unused DoNotReOrg::MissingHeadFinalizedCheckpoint variant.

4. Added two unit tests covering the divergence scenarios described in the issue:

   • store_checkpoint_used_for_finalization_age: stale store finality blocks reorg even when head's unrealized finality is fresh
   • ffg_competitiveness_only_checks_justification: matching justifications allow reorg despite differing finalized checkpoints

Closes #9359

[michaelsproul]

We're currently heavily reworking the reorg feature for Gloas:

• Enable late re-org and re-org interactive tests  #9405

Once the dust settles we'll ping you to fix merge conflicts, and then we'll review.

[VanshSahay]

@michaelsproul quick check, any changes to be made here?

[VanshSahay]

bumping this up @michaelsproul

[michaelsproul]

No bandwidth to review this at the moment sadly. Will revisit down the track.

As stated on the OG issue this is low prio (has no practical relevance)