fix(fault-proof): align catch-up head selection with upstream PR succinctlabs#927

Upstream replaced succinctlabs#926 with succinctlabs#927, adding two increments on top of the
same core selection logic:
- Emit a tracing::debug! when the canonical head is chosen from a
  catch-up chain outside the anchor subtree, so operators can tell the
  proposer is recovering on a non-descendant lineage.
- Cover the multi-chain case in a new unit test
  (multiple_catchup_chains_select_highest_tip): after repeated
  stall/recovery cycles, several genesis-rooted catch-up chains can sit
  in the cache, and the head must be the highest tip pooled across all
  of them, not the tip of the chain whose root sits highest.

Author: xzav3r

fault-proof/src/proposer.rs

@@ -273,6 +273,16 @@ impl ProposerState {
                 .cloned()
         });
 
+        // Log when the head comes from a catch-up chain so operators can tell the proposer is
+        // recovering along a chain outside the anchor subtree.
+        if let Some(head) = override_head.as_ref() {
+            tracing::debug!(
+                head_index = %head.index,
+                head_l2_block = %head.l2_block,
+                "Canonical head selected from a catch-up chain outside the anchor subtree"
+            );
+        }
+
         override_head.or(anchor_head)
     }
 }
@@ -2777,6 +2787,26 @@ mod tests {
             );
             assert_eq!(s.select_canonical_head().unwrap().index, U256::from(7));
         }
+
+        #[test]
+        fn multiple_catchup_chains_select_highest_tip() {
+            // Repeated stall/recovery cycles can leave several genesis-rooted catch-up chains in
+            // the cache at once. The head must be the highest-block tip across all qualifying
+            // chains pooled together (chain B's tip 9), even though chain B's root (block 250)
+            // sits below chain A's tip (block 300).
+            let anchor = game_with(5, 4, 100);
+            let s = state(
+                vec![
+                    anchor.clone(),
+                    game_with(6, u32::MAX, 200),
+                    game_with(7, 6, 300),
+                    game_with(8, u32::MAX, 250),
+                    game_with(9, 8, 400),
+                ],
+                Some(anchor),
+            );
+            assert_eq!(s.select_canonical_head().unwrap().index, U256::from(9));
+        }
     }
 
     async fn mock_prove(