Trigger backfill on startup if user switches to a supernode or semi-supernode (#8265)

Squashed commit of the following:

commit 2ed4f69
Author: Jimmy Chen <[email protected]>
Date:   Thu Oct 23 10:56:42 2025 +1100

    Change CLI CGC increase effective epoch to be based on head_epoch and apply other comment fixes.

commit 8cedbd1
Author: Jimmy Chen <[email protected]>
Date:   Thu Oct 23 00:20:17 2025 +1100

    Remove unnecessary changes to reduce diff.

commit 13cef6d
Author: Jimmy Chen <[email protected]>
Date:   Thu Oct 23 00:10:54 2025 +1100

    Minor clean ups.

commit 045afbb
Author: Jimmy Chen <[email protected]>
Date:   Wed Oct 22 23:59:01 2025 +1100

    Clean up and update function docs.

commit 6904556
Author: Jimmy Chen <[email protected]>
Date:   Wed Oct 22 23:51:17 2025 +1100

    Add column backfill when node's custody type changes via CLI.

commit 895e571
Author: Jimmy Chen <[email protected]>
Date:   Wed Oct 22 17:39:55 2025 +1100

    Add tests and TODOs

Author: jimmygchen

beacon_node/beacon_chain/src/builder.rs

@@ -931,18 +931,26 @@ where
 
         // Load the persisted custody context from the db and initialize
         // the context for this run
-        let custody_context = if let Some(custody) =
+        let (custody_context, cgc_changed_opt) = if let Some(custody) =
             load_custody_context::<E, THotStore, TColdStore>(store.clone())
         {
-            Arc::new(CustodyContext::new_from_persisted_custody_context(
+            let head_epoch = canonical_head
+                .cached_head()
+                .head_slot()
+                .epoch(E::slots_per_epoch());
+            CustodyContext::new_from_persisted_custody_context(
                 custody,
                 self.node_custody_type,
+                head_epoch,
                 &self.spec,
-            ))
+            )
         } else {
-            Arc::new(CustodyContext::new(self.node_custody_type, &self.spec))
+            (
+                CustodyContext::new(self.node_custody_type, &self.spec),
+                None,
+            )
         };
-        debug!(?custody_context, "Loading persisted custody context");
+        debug!(?custody_context, "Loaded persisted custody context");
 
         let beacon_chain = BeaconChain {
             spec: self.spec.clone(),
@@ -1019,7 +1027,7 @@ where
                     slot_clock,
                     self.kzg.clone(),
                     store,
-                    custody_context,
+                    Arc::new(custody_context),
                     self.spec,
                 )
                 .map_err(|e| format!("Error initializing DataAvailabilityChecker: {:?}", e))?,
@@ -1062,6 +1070,14 @@ where
             return Err(format!("Weak subjectivity verification failed: {:?}", e));
         }
 
+        if let Some(cgc_changed) = cgc_changed_opt {
+            // Update data column custody info if there's a CGC change from CLI flags.
+            // This will trigger column backfill.
+            let cgc_change_effective_slot =
+                cgc_changed.effective_epoch.start_slot(E::slots_per_epoch());
+            beacon_chain.update_data_column_custody_info(Some(cgc_change_effective_slot));
+        }
+
         info!(
             head_state = %head.beacon_state_root(),
             head_block = %head.beacon_block_root,