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

sdk/firstview/firstview.go

@@ -105,6 +105,16 @@ func (r *Reader) Picture(bucket string) *Picture {
 	return r.pics[bucket]
 }
 
+// KeyValue opens a followed state's picture by its bus name, where a process opened the bucket (process.BucketOpener,
+// as buildrun.FollowPlacement takes it); any other bucket is refused, for the reader reads its owners' states by their
+// first views alone.
+func (r *Reader) KeyValue(_ context.Context, bucket string) (jetstream.KeyValue, error) {
+	if p := r.Picture(bucket); p != nil {
+		return p, nil
+	}
+	return nil, fmt.Errorf("%s reads %s by its owner's first view alone, and does not follow it", r.Module, bucket)
+}
+
 // Followers are the followers, in the order they were added.
 func (r *Reader) Followers() []*views.Follower {
 	r.mu.Lock()