test(consensus): pin the fork digest to other clients' output

A fork digest that differs by one byte is invisible locally but fatal on the
wire: peers reject our Status and we subscribe to the wrong gossip topics.
Cover the EIP-7892 masking against Lighthouse's own vector, plus the two
boundary behaviours that are easy to regress — the digest only changing on a
BPO entry, and no masking at all before Fulu.

Author: perfogic

crates/common/consensus/misc/src/fork_data.rs

@@ -52,3 +52,70 @@ pub fn compute_fork_digest(
 
     B32::from_slice(&result)
 }
+
+#[cfg(test)]
+mod tests {
+    use alloy_primitives::fixed_bytes;
+
+    use super::*;
+
+    fn schedule() -> Vec<BlobParameters> {
+        vec![
+            BlobParameters {
+                epoch: 9,
+                max_blobs_per_block: 9,
+            },
+            BlobParameters {
+                epoch: 100,
+                max_blobs_per_block: 100,
+            },
+            BlobParameters {
+                epoch: 150,
+                max_blobs_per_block: 175,
+            },
+        ]
+    }
+
+    fn fork_data() -> ForkData {
+        ForkData {
+            current_version: fixed_bytes!("0x06000000"),
+            genesis_validators_root: B256::ZERO,
+        }
+    }
+
+    /// Cross-check against the vector in Lighthouse's `blob_schedule_fork_digest`
+    /// (`consensus/types/src/core/chain_spec.rs`). A mismatch means peers reject our
+    /// `Status` and we subscribe to the wrong gossip topics, so this must stay
+    /// byte-identical to other clients.
+    #[test]
+    fn test_compute_fork_digest_matches_other_clients() {
+        assert_eq!(
+            compute_fork_digest(fork_data(), &schedule(), 100, 100),
+            fixed_bytes!("0xdf67557b"),
+        );
+    }
+
+    /// The digest only changes on a BPO boundary, so every epoch between two entries
+    /// resolves to the earlier one.
+    #[test]
+    fn test_compute_fork_digest_is_stable_between_bpo_entries() {
+        assert_eq!(
+            compute_fork_digest(fork_data(), &schedule(), 100, 149),
+            compute_fork_digest(fork_data(), &schedule(), 100, 100),
+        );
+        assert_ne!(
+            compute_fork_digest(fork_data(), &schedule(), 100, 150),
+            compute_fork_digest(fork_data(), &schedule(), 100, 149),
+        );
+    }
+
+    /// Before Fulu there is no BPO masking, so the digest is the first four bytes of the
+    /// fork data root.
+    #[test]
+    fn test_compute_fork_digest_is_unmasked_before_fulu() {
+        assert_eq!(
+            compute_fork_digest(fork_data(), &schedule(), 100, 99),
+            B32::from_slice(&fork_data().compute_fork_data_root()[..4]),
+        );
+    }
+}