Add optional EIP-8025 execution proofs

[frisitano]

Description

Adds optional EIP-8025 execution proof support on top of the updated unstable branch.

This wires proof types, proof-engine client integration, proof status tracking, execution proof gossip/RPC, HTTP pool submission, and proof catch-up sync. Proof validity remains optional for fork choice: payload validity is not flipped to valid unless normal execution validation succeeds, with proof-backed promotion guarded behind non-default quorum configuration.

Additional Info

Base branch eth-act/unstable was updated to upstream/unstable at dfb2591 before opening this draft.

Validation run locally:

• cargo fmt --all
• env -u RUSTC_WRAPPER cargo test -p beacon_chain latest_status_with_valid_proofs_ignores_empty_and_unconfigured_statuses
• env -u RUSTC_WRAPPER cargo test -p network servable_missing_proofs_starts_at_finalized_slot --lib
• env -u RUSTC_WRAPPER cargo check -p network
• In a temporary old optional-proofs worktree, test::test_proof_engine_sync passed with the equivalent requester-side finalized-window clamp applied, then the temporary patch was reverted

Draft because the migrated simulator proof-engine test crate is not yet present on this branch and the proof-sync design still needs review.