Clarify artifact reader ownership transfer

types.go

@@ -295,15 +295,17 @@ type Artifact struct {
 //
 // Read may be called concurrently. It must honor ctx cancellation to make
 // Prepare, PrepareExecution, and Run responsive to cancellation. The engine
-// passes a copied ref and immediately copies the returned Artifact.Body; it
-// does not retain either value. Readers supply artifact metadata, and the
+// passes a copied ref and defensively copies the returned Artifact.Body.
+// A successful Read transfers the returned artifact and its mutable body
+// storage to Promptkit. The reader must not mutate them through retained
+// aliases after return. Private buffers may be reused only when the returned
+// storage is independent. Readers supply artifact metadata, and the
 // engine assigns an input-map name only when the returned artifact name is
 // empty.
 //
 // An injected reader owns any application-specific path containment,
 // authorization, content-size, and content-type policy. It must protect
 // sensitive references and bodies in its logging and in any copies it retains.
-// It may reuse or mutate the returned artifact and body after Read returns.
 //
 // Returning a non-nil error makes the engine return an error matching
 // ErrArtifactLoad while preserving the reader error through errors.Is.