feat(fork-choice): implement ForkChoice orchestrator struct (task 12)

Add ForkChoice wrapping ProtoArray, Votes, and checkpoint state with
full public API: onBlock, onAttestation, getHead, setProposerBoost,
onAttesterSlashing, prune, validateLatestHash, and Gloas ePBS methods
(onExecutionPayload, notifyPtcMessages). Pre-allocated DeltasCache
eliminates per-slot allocation in the getHead hot path.

Author: GrapeBaBa