expected_current_slot and slot_timestamp hardcode 12-second slots

[lovon-spec]

ConsensusSpec parameterizes every consensus constant that differs between presets except the slot duration. Three sites assume 12 seconds (master @ 43a8c9f):

• helios/ethereum/consensus-core/src/consensus_core.rs

  Lines 389 to 395 in 43a8c9f

  	pub fn expected_current_slot(now: SystemTime, genesis_time: u64) -> u64 {
  	let now = now.duration_since(UNIX_EPOCH).unwrap_or_default().as_secs();
  	let since_genesis = now - genesis_time;
  	since_genesis / 12
  	}

  — expected_current_slot computes since_genesis / 12

• helios/ethereum/src/consensus.rs

  Lines 660 to 662 in 43a8c9f

  	fn slot_timestamp(&self, slot: u64) -> u64 {
  	slot * 12 + self.config.chain.genesis_time
  	}

  — Inner::slot_timestamp computes slot * 12; used by is_valid_checkpoint and the head-age log line

• helios/ethereum/src/consensus.rs

  Line 178 in 43a8c9f

  let mut interval = interval_at(start, std::time::Duration::from_secs(12));

  — the advance loop ticks every Duration::from_secs(12), and

  helios/ethereum/src/consensus.rs

  Line 531 in 43a8c9f

  let time_to_next_slot = next_slot_timestamp - now;

  phases each tick 4 s (one third of a 12 s slot) into the slot

A ConsensusSpec for a chain with a different slot time therefore gets wrong wall-clock slots. On a 5-second chain (Gnosis), expected_current_slot is 2.4x too low, so verify_generic_update rejects every fresh update with InvalidTimestamp; and slot_timestamp ages checkpoints 2.4x too fast, so a checkpoint that is 6 days old fails the max_checkpoint_age check as 14.4 days old. The minimal preset the test vectors under testdata/ are built from also declares SECONDS_PER_SLOT: 6, which MinimalConsensusSpec cannot express.

Proposed fix: a ConsensusSpec::seconds_per_slot() default method returning 12, used at those three sites, with MinimalConsensusSpec overriding to 6. Mainnet is unaffected. PR: #809.