Process Electra and Single attestations in the slasher

[rocker-zhang]

Summary

The slasher was silently dropping Attestation::Electra and Attestation::Single from its detection path, so attester-slashing detection has been effectively dead on mainnet since the Electra fork.

This change unifies the slasher's internal IndexedAttestation and AttesterSlashing types on the Electra form (Electra's bound supersets Phase 0's, and the live network is post-Electra). The three attestation kinds are handled as follows:

• Attestation::Single: built directly from attester_index into an Electra IndexedAttestation via ContiguousList::try_from_iter.
• Attestation::Electra: passed through electra::get_indexed_attestation.
• Attestation::Phase0: widened to Electra via ContiguousList::try_from_iter (Phase 0's MaxValidatorsPerCommittee bound is a subset of Electra's MaxAttestersPerSlot).

process_attester_slashing now emits CombinedAttesterSlashing::Electra. The change ripples through slasher/src/{attestations,indexed_attestations,messages,status,targets}.rs, validator/src/validator.rs, and a 3-line comment in block_producer/src/block_producer.rs documenting the intentional pre-Electra filter drop.

Tests

Three new tests:

• single_attestation_converts_to_single_index_indexed_attestation (slasher.rs): Single → Electra IndexedAttestation round-trip.
• phase0_indexed_attestation_widens_preserving_indices_data_and_signature (slasher.rs): Phase 0 → Electra widening preserves all fields.
• detects_double_vote_for_each_attester_in_multi_index_attestation (attestations.rs): multi-attester double-vote detection path.

cargo test -p slasher: 9/9 pass. cargo check -p validator: clean.

Compat notes

• Fork choice and block production re-validate independently, so the always-Electra-output choice does not affect their correctness.
• AttestationData is fork-independent, so existing slasher DB entries remain byte-compatible.
• SSZ encoding of an Electra IndexedAttestation containing a single index has the same on-wire shape as a Phase 0 IndexedAttestation with the same index, modulo the attesting_indices type bound.

Fixes #655.