The watched mark is no longer ours alone

This engine has written `seen` into the shared shape all along, on the
grounds that the iPhone had no unread mark on a story. That turned out to
be wrong in a way worth recording: the iPhone HAS the mark and was not
writing it, locally or to a backup, so the ring around every contact
relit at each launch and a restore through it lost the lot.

The iPhone writes it now, so the comment claiming the field is ours alone
is false and is replaced, and `stories[].seen` moves out of the vector's
divergence list and into the canonical set.

No behaviour change here. The code was already right; only what it said
about the other engine was not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

engine/src/main/kotlin/io/flamenet/e2e/BackupPayload.kt

@@ -280,9 +280,12 @@ object BackupPayload {
             s.optString("key").ifEmpty { s.optString("id") }.ifEmpty { s.optLong("at").toString() },
         )
         s.optJSONObject("set")?.let { out.put("set", it) }
-        // The iPhone has no unread mark on a story, so this is ours alone. It
-        // travels anyway: coming back here it is the difference between a strip
-        // that remembers what you watched and one that shows everything as new.
+        // Whether the strip has been watched. This used to be ours alone and
+        // travelled anyway, on the grounds that it was the difference between a
+        // strip remembering what you watched and one showing everything as new.
+        // The iPhone turned out to HAVE the mark and not to be writing it, so
+        // it relit every ring at each launch; both engines carry it now and it
+        // is in the canonical set rather than a divergence.
         if (s.optBoolean("seen", false)) out.put("seen", true)
         return out
     }