feat(base,learn): suggest changes within a shared studyset

ADR 0044's step 6b (D44, D46 to D48): a Viewer, and an agent, suggest
changes within a shared studyset, which its Owner and Editors accept or
decline.

A suggestion or report is written as Explore's are, in its writer's own
synced space, with kinds of its own (space-suggestion, space-report)
and the space's id in feedback_thread's publicationId, so no synced
table changes and older apps, skipping kinds they don't know, keep
syncing. Where the studyset is private it is sealed with its key, bound
to the space; a reply to it is sealed for its thread.

Learn's server gathers a space's threads for its members alone
(feedback.threadsInSpace, NotFound for anyone else): only threads and
replies by members now, decisions only from rows in the space itself,
where the engine lets its Owner and Editors alone write; sealed
payloads pass in base64 with their key and nonce. The inbox adds the
threads its caller decides on or wrote. The module tells a host a
person's spaces and each space's members (sharedSpacesOf,
membersOfSpaces). No migration: no table changes.

In the app, a Viewer's card and note offer Suggest a change: the fields
write into the suggestion alone, sent with a message. A shared
studyset's menu lists its Suggestions, where its Owner and Editors
accept a change as a revision, or a new card or note into the
studyset, and decide in the space. A suggestion naming a file that
isn't the studyset's, or bringing files of its own, isn't taken.

Whats-New: In a studyset shared with you as a viewer, suggest a change to a card or note; its owner and editors find your suggestions under Suggestions in its menu and accept or decline them.
Whats-New-De: In einem Lernset, das du nur ansehen darfst, schlägst du Änderungen an Karteikarten und Notizen vor; Besitzer:in und Bearbeiter:innen finden sie im Menü unter „Vorschläge“ und nehmen sie an oder lehnen sie ab.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/test/integration/shared_space/shared_space_test.dart

@@ -485,5 +485,29 @@ void main() {
 
       expect(await SharedSpaceInvitation.db.count(session), 0);
     });
+
+    test("a host asks a person's spaces and each space's members, with "
+        'their roles; nobody else is named', () async {
+      final space = await shared();
+      final bobs = (await api.create(signedIn(bob), 'studyset')).id;
+
+      expect(await sharedSpacesOf(session, carol), {
+        space: SharedSpaceRole.viewer,
+      });
+      expect(await sharedSpacesOf(session, bob), {
+        space: SharedSpaceRole.editor,
+        bobs: SharedSpaceRole.owner,
+      });
+      expect(await membersOfSpaces(session, {space, bobs}), {
+        space: {
+          alice: SharedSpaceRole.owner,
+          bob: SharedSpaceRole.editor,
+          carol: SharedSpaceRole.viewer,
+        },
+        bobs: {bob: SharedSpaceRole.owner},
+      });
+      expect(await membersOfSpaces(session, {const Uuid().v7obj()}), isEmpty);
+      expect(await membersOfSpaces(session, {}), isEmpty);
+    });
   });
 }