trie/bintrie: move dirty+mustRecompute flip into setValue

The invariant "mutating a value slot must mark the stem for re-hash
and re-flush" was enforced by every caller remembering to set both
flags after setValue. Moving the flip into setValue itself makes it
structurally impossible to forget, and drops the duplicate flag-sets
at each callsite.

decodeNode's on-disk load path still writes directly to sn.values
because loaded stems must retain whatever mustRecompute/dirty state
the caller asked for (typically both false).

trie/bintrie/stem_node.go

@@ -51,8 +51,15 @@ func (sn *StemNode) allValues() [][]byte {
 	return sn.values[:]
 }
 
+// setValue mutates a value slot and marks the stem for re-hash and
+// re-flush. This is the only API for post-load value mutation; direct
+// values[...] writes are reserved for the on-disk load path in
+// decodeNode, which must leave mustRecompute/dirty at their loaded
+// state.
 func (sn *StemNode) setValue(suffix byte, value []byte) {
 	sn.values[suffix] = value
+	sn.mustRecompute = true
+	sn.dirty = true
 }
 
 func (sn *StemNode) Hash() common.Hash {

trie/bintrie/store_ops.go

@@ -199,12 +199,10 @@ func (s *NodeStore) insertValuesAtStem(ref nodeRef, stem []byte, values [][]byte
 	case kindStem:
 		sn := s.getStem(ref.Index())
 		if sn.Stem == [StemSize]byte(stem[:StemSize]) {
-			// Same stem — merge values
+			// Same stem — merge values (setValue marks dirty+mustRecompute)
 			for i, v := range values {
 				if v != nil {
 					sn.setValue(byte(i), v)
-					sn.mustRecompute = true
-					sn.dirty = true
 				}
 			}
 			return ref, nil
@@ -233,7 +231,8 @@ func (s *NodeStore) insertValuesAtStem(ref nodeRef, stem []byte, values [][]byte
 		return s.insertValuesAtStem(resolved, stem, values, resolver, depth)
 
 	case kindEmpty:
-		// Create new StemNode
+		// Create new StemNode. Flag flips before the value loop so an
+		// all-nil values input still marks the newly-created stem dirty.
 		stemIdx := s.allocStem()
 		sn := s.getStem(stemIdx)
 		copy(sn.Stem[:], stem[:StemSize])
@@ -242,7 +241,7 @@ func (s *NodeStore) insertValuesAtStem(ref nodeRef, stem []byte, values [][]byte
 		sn.dirty = true
 		for i, v := range values {
 			if v != nil {
-				sn.values[i] = v
+				sn.setValue(byte(i), v)
 			}
 		}
 		return makeRef(kindStem, stemIdx), nil