Permit safe cross-run output declarations

docs/internal/artifacts.md

@@ -249,6 +249,12 @@ New publication creates run-scoped immutable objects, including
 mutable selector is `current/commit-pointer.json`; readers verify its selected
 commit and declared object generations/checksums.
 
+A `published_output` declaration may identify its commit's run or an earlier
+immutable run in the same campaign/session namespace. The key parser validates
+that namespace, rejects mutable and malformed keys, and derives the originating
+run from the exact declaration. All other commit artifacts remain confined to
+the commit's own run prefix.
+
 Campaign, session, and Narratio run IDs are validated as portable opaque
 segments at configuration and artifact boundaries before they can be used in a
 workspace or S3 namespace. Previous-artifact destinations remain typed,