feat(blockchain): pre-build proposer block one interval early

CLAUDE.md

@@ -47,17 +47,17 @@ crates/
 
 ### Tick-Based Validator Duties (4-second slots, 5 intervals per slot)
 ```
-Interval 0: Block proposal → accept attestations if proposal exists
+Interval 0: Block published (at the slot boundary). The build+publish code path is merged into the previous slot's interval 4 (see below) and aligned to publish here; no attestation acceptance happens at interval 0.
 Interval 1: Attestation production (all validators, including proposer)
 Interval 2: Aggregation (aggregators create proofs from gossip signatures)
 Interval 3: Safe target update (fork choice)
-Interval 4: Accept accumulated attestations
+Interval 4: Accept accumulated attestations; build the NEXT slot's block and publish it aligned to that slot's interval 0 (build and publish merged into this tick)
 ```
 
 ### Attestation Pipeline
 ```
 Gossip → Signature verification → new_attestations (pending)
-  ↓ (intervals 0/4)
+  ↓ (interval 4)
 promote → known_attestations (fork choice active)
   ↓
 Fork choice head update

crates/blockchain/src/block_builder.rs

@@ -43,6 +43,18 @@ pub struct PostBlockCheckpoints {
     pub finalized: Checkpoint,
 }
 
+/// Whether a pre-build that has just finished has already run into (or past)
+/// the start of its target slot — i.e. into the interval-0 publish window.
+///
+/// When true, the overrun-catch-up in `handle_tick` will have skipped that
+/// slot's interval-0 proposal tick, so the freshly built block must be
+/// published in place rather than stashed for a tick that will never fire.
+///
+/// Pure so it can be unit-tested without an actor or store.
+pub(crate) fn build_overran_publish_window(now_ms: u64, genesis_time_ms: u64, slot: u64) -> bool {
+    now_ms >= genesis_time_ms + slot * crate::MILLISECONDS_PER_SLOT
+}
+
 /// Build a valid block on top of this state.
 ///
 /// Selects attestations via `select_attestations`, compacts duplicate
@@ -1336,3 +1348,24 @@ mod tests {
         assert_eq!(covered, HashSet::from([0, 1, 2, 3]));
     }
 }
+
+#[cfg(test)]
+mod prebuild_tests {
+    use super::*;
+
+    #[test]
+    fn overran_only_once_now_reaches_slot_start() {
+        let genesis = 1000;
+        let slot = 10;
+        let slot_start = genesis + slot * crate::MILLISECONDS_PER_SLOT;
+        // Build finished before the target slot opened: stash for interval 0.
+        assert!(!build_overran_publish_window(slot_start - 1, genesis, slot));
+        // Build finished exactly at / past the slot start: publish in place.
+        assert!(build_overran_publish_window(slot_start, genesis, slot));
+        assert!(build_overran_publish_window(
+            slot_start + 5_000,
+            genesis,
+            slot
+        ));
+    }
+}