DLV-M2: the checker follows the indexer's candidates, modules, manifests, packages and predefined keys and the assigner's placement by first views

ADR 0615 §1 and ADR 0596: the checker invokes the six read tools, consumes their views families and
reads none of the six buckets; this ends its direct read of the packages state (the lead's ruling 2).
It keeps first-views and findings and announces both as its own. The lead's ruling: an owner that is
down never holds the start; the checks, the tries and the hand asks start once every first view is
answered, the placement is followed into the toolchain gate once its first view is. firstview.Reader
opens a followed state's picture as a process.BucketOpener (the builder uses it too). Tools
checker_first_views and checker_first_view_again.

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go

@@ -178,7 +178,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 			return
 		case <-placement.Follower().Ready():
 		}
-		pe, err := buildrun.FollowPlacement(ctx, pictures{placement}, gate, b.NodesChanged, logger.Printf)
+		pe, err := buildrun.FollowPlacement(ctx, reader, gate, b.NodesChanged, logger.Printf)
 		if err != nil {
 			placementErrs <- err
 			return
@@ -270,16 +270,6 @@ const (
 	candidatesState = "candidates"
 )
 
-// pictures opens the placement's picture where the gate's follower opens its bucket.
-type pictures struct{ placement *firstview.Picture }
-
-func (p pictures) KeyValue(ctx context.Context, bucket string) (jetstream.KeyValue, error) {
-	if bucket != p.placement.Bucket() {
-		return nil, fmt.Errorf("the builder reads %s by its owner's first view alone", bucket)
-	}
-	return p.placement, nil
-}
-
 // buildFromModules builds from the indexer's modules and candidates once the first views of the modules, the manifests and
 // the candidates are answered (never at the start: an indexer that is down holds no other part of it), and follows them.
 func buildFromModules(ctx context.Context, b *build.Builder, modules, manifests, candidates *firstview.Picture) error {