Align legacy post snapshots with bDS2.

specs/post.allium

@@ -107,9 +107,9 @@ entity Post {
     updated_at: Timestamp
     published_at: Timestamp?
 
-    -- Published snapshot: copy of title/content/tags/categories/excerpt as of
-    -- the last publish. Used by changes_affect_published_content to decide when
-    -- an edit reopens a published post to draft (see ReopenPublishedPost).
+    -- Legacy bDS2-compatible columns. Publishing does not populate them and
+    -- application behaviour does not depend on them; imported non-null values
+    -- remain passive database data.
     published_title: String?
     published_content: String?
     published_tags: String?
@@ -200,6 +200,9 @@ rule ReopenPublishedPost {
     when: UpdatePostRequested(post, changes)
     requires: post.status = published
     requires: changes_affect_published_content(changes)
+        -- Compares supplied metadata with the current post fields and supplied
+        -- content with the canonical file body. Legacy published_* columns are
+        -- not consulted because bDS2 does not populate them.
     ensures: post.status = draft
 }