feat(cli): add offline block-building benchmark sub-command

`ethlambda benchmark synthetic` measures block building exactly as
executed when the node proposes, against a reproducible synthetic
workload, with no devnet required. "Optimize block building" (#465) is
the top roadmap item, but the only observability today is Prometheus
histograms on a live devnet: noisy, not reproducible, and unable to
compare an optimization against a baseline.

The harness drives the production proposer entry point
(`produce_block_with_signatures`) over a seeded in-memory chain, seeding
the pending pool per slot and letting the proposal tick promote it, as on
a live node. Per-phase timings come from the existing
`lean_block_proposal_attestation_build_phase_seconds` histogram: the
per-label sample sums are deltaed between iterations, so attribution is
exact and the hot path is untouched. Each iteration reports its block
root, so a baseline-vs-optimized diff proves an optimization changed only
speed and not attestation selection.

Dispatch goes through the `benchmark` token that command.rs already
strips, parsed by a `clap::Parser` of its own, so the harness arguments
never enter CliOptions and the node parser keeps the exact shape it has
today. `main` becomes synchronous and only the node path enters the tokio
runtime: the benchmark is synchronous CPU-bound work and would otherwise
park a worker thread for its whole run. Its logs go to stderr so the
report owns stdout.

NEW_PAYLOAD_CAP becomes public so the harness can reject a
--proofs-per-data batch the pending pool would evict whole. `make bench`
runs it.

Reports one row per measured iteration. Aggregate statistics, build
provenance and machine-readable output follow separately, as does the
real-crypto workload — see docs/plans/block-building-benchmark.md for the
milestones.

Author: pablodeymo

Cargo.lock

@@ -1,4 +1,4 @@
-.PHONY: help fmt lint docker-build shadow-build shadow-docker-build run-devnet test docs docs-deps docs-serve
+.PHONY: help fmt lint bench docker-build shadow-build shadow-docker-build run-devnet test docs docs-deps docs-serve
 
 help: ## 📚 Show help for each of the Makefile recipes
 	@grep -E '^[a-zA-Z0-9_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'
@@ -14,6 +14,11 @@ test: leanSpec/fixtures ## 🧪 Run all tests
 	# signature verification/aggregation, without paying for LTO on every rebuild
 	cargo test --workspace --profile release-fast
 
+BENCH_ARGS ?= synthetic --mock-crypto
+
+bench: ## 🏁 Benchmark block building offline (override BENCH_ARGS to customize)
+	cargo run --release --bin ethlambda -- benchmark $(BENCH_ARGS)
+
 GIT_COMMIT=$(shell git rev-parse HEAD)
 GIT_BRANCH=$(shell git rev-parse --abbrev-ref HEAD)
 DOCKER_TAG?=local

Makefile

@@ -21,6 +21,7 @@ shadow-integration = ["ethlambda-crypto/shadow-integration"]
 [dependencies]
 ethlambda-blockchain.workspace = true
 ethlambda-crypto.workspace = true
+ethlambda-metrics.workspace = true
 ethlambda-network-api.workspace = true
 ethlambda-p2p.workspace = true
 ethlambda-types.workspace = true

bin/ethlambda/Cargo.toml

@@ -0,0 +1,145 @@
+//! Synthetic benchmark corpus: deterministic validators, a genesis store, and
+//! per-slot attestation-pool seeding.
+
+use std::sync::Arc;
+
+use ethlambda_blockchain::store::produce_attestation_data;
+use ethlambda_storage::{Store, backend::InMemoryBackend};
+use ethlambda_types::{
+    attestation::{AggregationBits, HashedAttestationData},
+    block::SingleMessageAggregate,
+    state::{State, Validator, ValidatorPubkeyBytes},
+};
+
+/// Fixed genesis time for synthetic runs. The harness derives every tick
+/// timestamp from slot numbers relative to this value and never reads the wall
+/// clock, so runs are reproducible at any time of day.
+const GENESIS_TIME: u64 = 1_700_000_000;
+
+pub(crate) struct SyntheticCorpus {
+    num_validators: u64,
+    proofs_per_data: u64,
+}
+
+impl SyntheticCorpus {
+    pub(crate) fn new(num_validators: u64, proofs_per_data: u64) -> Self {
+        Self {
+            num_validators,
+            proofs_per_data,
+        }
+    }
+
+    /// Build a genesis store over an in-memory backend with `num_validators`
+    /// seed-derived validators.
+    ///
+    /// Pubkeys are deterministic placeholder bytes: in mock-crypto mode no code
+    /// path decodes them (signature verification is skipped and best-proof
+    /// compaction never resolves pubkeys).
+    pub(crate) fn genesis_store(&self, seed: u64) -> Store {
+        let mut rng_state = seed;
+        let validators = (0..self.num_validators)
+            .map(|index| Validator {
+                attestation_pubkey: synthetic_pubkey(&mut rng_state),
+                proposal_pubkey: synthetic_pubkey(&mut rng_state),
+                index,
+            })
+            .collect();
+        let genesis_state = State::from_genesis(GENESIS_TIME, validators);
+        Store::from_anchor_state(Arc::new(InMemoryBackend::new()), genesis_state)
+    }
+
+    /// Seed the pending ("new") pool with the full validator set's attestations
+    /// for `attestation_slot`, split into `proofs_per_data` disjoint aggregates.
+    ///
+    /// Mirrors what committee aggregators gossip during a slot: several
+    /// aggregates for the same `AttestationData`, each covering a validator
+    /// subset. The proposal tick then promotes them to the known pool, exactly
+    /// as on a live node. Entries are inserted in a fixed order because pool
+    /// insertion order pins within-entry proof choice during selection.
+    pub(crate) fn seed_pool(&self, store: &mut Store, attestation_slot: u64) {
+        let data = produce_attestation_data(store, attestation_slot);
+        let entries = participant_groups(self.num_validators, self.proofs_per_data)
+            .into_iter()
+            .map(|participants| {
+                (
+                    HashedAttestationData::new(data.clone()),
+                    SingleMessageAggregate::empty(participants),
+                )
+            })
+            .collect();
+        store.insert_new_aggregated_payloads_batch(entries);
+    }
+}
+
+/// Partition validators 0..num_validators into `groups` disjoint bitfields,
+/// assigning validator `i` to group `i % groups`. Every group is non-empty
+/// (groups is capped at the validator count) and the union covers every
+/// validator exactly once.
+fn participant_groups(num_validators: u64, groups: u64) -> Vec<AggregationBits> {
+    let groups = groups.clamp(1, num_validators);
+    (0..groups)
+        .map(|group| {
+            let mut bits = AggregationBits::with_length(num_validators as usize)
+                .expect("validator count is within the bitlist limit");
+            for index in (group..num_validators).step_by(groups as usize) {
+                bits.set(index as usize, true)
+                    .expect("index is within the bitlist length");
+            }
+            bits
+        })
+        .collect()
+}
+
+/// splitmix64: tiny deterministic generator for placeholder pubkey bytes,
+/// avoiding a rand dependency.
+fn splitmix64(state: &mut u64) -> u64 {
+    *state = state.wrapping_add(0x9e37_79b9_7f4a_7c15);
+    let mut z = *state;
+    z = (z ^ (z >> 30)).wrapping_mul(0xbf58_476d_1ce4_e5b9);
+    z = (z ^ (z >> 27)).wrapping_mul(0x94d0_49bb_1331_11eb);
+    z ^ (z >> 31)
+}
+
+fn synthetic_pubkey(rng_state: &mut u64) -> ValidatorPubkeyBytes {
+    let mut bytes = [0u8; 52];
+    for chunk in bytes.chunks_mut(8) {
+        let word = splitmix64(rng_state).to_le_bytes();
+        chunk.copy_from_slice(&word[..chunk.len()]);
+    }
+    bytes
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ethlambda_types::attestation::validator_indices;
+
+    #[test]
+    fn participant_groups_partition_all_validators() {
+        for (validators, groups) in [(8u64, 2u64), (8, 3), (5, 8), (1, 1), (4096, 4)] {
+            let partition = participant_groups(validators, groups);
+            assert_eq!(partition.len() as u64, groups.min(validators));
+            let mut seen = vec![0u32; validators as usize];
+            for bits in &partition {
+                let indices: Vec<u64> = validator_indices(bits).collect();
+                assert!(!indices.is_empty(), "every group must be non-empty");
+                for index in indices {
+                    seen[index as usize] += 1;
+                }
+            }
+            assert!(
+                seen.iter().all(|&count| count == 1),
+                "every validator must appear in exactly one group: {seen:?}"
+            );
+        }
+    }
+
+    #[test]
+    fn synthetic_pubkeys_are_deterministic() {
+        let mut a = 42u64;
+        let mut b = 42u64;
+        assert_eq!(synthetic_pubkey(&mut a), synthetic_pubkey(&mut b));
+        let mut c = 43u64;
+        assert_ne!(synthetic_pubkey(&mut a), synthetic_pubkey(&mut c));
+    }
+}