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>

apps/learn/halkyon_learn_flutter/test/support/fake_feedback_gateways.dart

@@ -2,22 +2,35 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_learn_flutter/src/feature/feedback/data/feedback_envelopes.dart';
 import 'package:halkyon_learn_flutter/src/feature/feedback/data/feedback_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/feedback/model/feedback.dart';
 
 /// A [FeedbackWriteGateway] in memory: what was written, in order.
 class FakeFeedbackWriteGateway() implements FeedbackWriteGateway {
-  /// The threads written: their published studyset, kind and payload JSON.
+  /// The threads written: their published studyset, kind and payload JSON
+  /// (opened, where [FakeFeedbackEnvelopes] sealed it), and within a shared
+  /// space, the space, its row id and its key.
   final threads =
       <
-        ({String publicationId, FeedbackKind kind, Map<String, Object?> json})
+        ({
+          String publicationId,
+          FeedbackKind kind,
+          Map<String, Object?> json,
+          String? space,
+          String? id,
+          String? keyId,
+        })
       >[];
 
-  /// The replies written, by thread.
-  final messages = <({String threadId, Map<String, Object?> json})>[];
+  /// The replies written, by thread, with their key.
+  final messages =
+      <({String threadId, Map<String, Object?> json, String? keyId})>[];
 
-  /// The decisions written, by thread.
-  final decisions = <({String threadId, FeedbackDecision decision})>[];
+  /// The decisions written, by thread, and the shared space they went in.
+  final decisions =
+      <({String threadId, FeedbackDecision decision, String? space})>[];
 
   /// When set, writing a decision fails with it.
   Exception? failDecisions;
@@ -30,29 +43,43 @@ class FakeFeedbackWriteGateway() implements FeedbackWriteGateway {
     required String publicationId,
     required FeedbackKind kind,
     required ByteData payload,
+    String? space,
+    String? id,
+    String? keyId,
+    ByteData? nonce,
   }) async {
     threads.add((
       publicationId: publicationId,
       kind: kind,
       json: _json(payload),
+      space: space,
+      id: id,
+      keyId: keyId,
     ));
-    return 'thread-${threads.length}';
+    return id ?? 'thread-${threads.length}';
   }
 
   @override
-  Future<void> insertMessage(String threadId, ByteData payload) async =>
-      messages.add((threadId: threadId, json: _json(payload)));
+  Future<void> insertMessage(
+    String threadId,
+    ByteData payload, {
+    String? id,
+    String? keyId,
+    ByteData? nonce,
+  }) async =>
+      messages.add((threadId: threadId, json: _json(payload), keyId: keyId));
 
   @override
   Future<void> insertDecision(
     String threadId,
-    FeedbackDecision decision,
-  ) async {
+    FeedbackDecision decision, {
+    String? space,
+  }) async {
     await holdDecisions?.future;
     if (failDecisions case final error?) {
       throw error;
     }
-    decisions.add((threadId: threadId, decision: decision));
+    decisions.add((threadId: threadId, decision: decision, space: space));
   }
 
   static Map<String, Object?> _json(ByteData payload) =>
@@ -96,6 +123,15 @@ class FakeFeedbackServerGateway({
     ];
   }
 
+  @override
+  Future<List<FeedbackThread>> threadsInSpace(String space) async {
+    await _maybeFail();
+    return [
+      for (final thread in _threads)
+        if (thread.space == space) thread,
+    ];
+  }
+
   @override
   Future<List<FeedbackThread>> inbox() async {
     await _maybeFail();
@@ -131,3 +167,61 @@ class FakeFeedbackServerGateway({
     return files[sha256]!;
   }
 }
+
+/// [FeedbackEnvelopes] in memory: the studysets [spaces] holds are shared
+/// in those spaces, those [keys] holds private under those keys; sealing
+/// marks a payload with its key and leaves it readable, so tests read what
+/// was written, and opening refuses a key not in [keys]'s values or a row
+/// sealed for another.
+class FakeFeedbackEnvelopes({
+  final Map<String, String> spaces = const {},
+  final Map<String, String> keys = const {},
+  final Map<String, String> titles = const {},
+}) implements FeedbackEnvelopes {
+  var _ids = 0;
+
+  @override
+  Future<String?> spaceOf(String studysetId) async => spaces[studysetId];
+
+  @override
+  Future<String?> keyToWriteIn(String studysetId) async => keys[studysetId];
+
+  @override
+  String newId() => 'row-${++_ids}';
+
+  @override
+  Envelope seal(
+    ByteData plain, {
+    required String? keyId,
+    required String table,
+    required String id,
+    required String bound,
+  }) => (
+    keyId: keyId,
+    nonce: keyId == null
+        ? null
+        : ByteData.sublistView(utf8.encode('$table|$id|$bound')),
+    payload: plain,
+  );
+
+  @override
+  ByteData? open(
+    Envelope envelope, {
+    required String table,
+    required String id,
+    required String bound,
+  }) {
+    if (envelope.keyId case final keyId?) {
+      final nonce = envelope.nonce;
+      if (!keys.containsValue(keyId) ||
+          nonce == null ||
+          utf8.decode(Uint8List.sublistView(nonce)) != '$table|$id|$bound') {
+        return null;
+      }
+    }
+    return envelope.payload;
+  }
+
+  @override
+  Future<String> titleOf(String studysetId) async => titles[studysetId] ?? '';
+}

apps/learn/halkyon_learn_flutter/test/support/test_dependencies.dart

@@ -39,6 +39,7 @@ import 'package:halkyon_learn_flutter/src/feature/explore/data/explore_repositor
 import 'package:halkyon_learn_flutter/src/feature/explore/data/release_copier.dart';
 import 'package:halkyon_learn_flutter/src/feature/export/data/export_archive.dart';
 import 'package:halkyon_learn_flutter/src/feature/export/data/studyset_markdown.dart';
+import 'package:halkyon_learn_flutter/src/feature/feedback/data/feedback_envelopes.dart';
 import 'package:halkyon_learn_flutter/src/feature/feedback/data/feedback_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/feedback/data/feedback_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/geo_map/data/geo_map_repository.dart';
@@ -162,6 +163,7 @@ Dependencies testDependencies({
   ExploreGateway? exploreGateway,
   FeedbackWriteGateway? feedbackWriteGateway,
   FeedbackServerGateway? feedbackServerGateway,
+  FeedbackEnvelopes? feedbackEnvelopes,
   DiscussionGateway? discussionGateway,
   SharedSpaceGateway? sharedSpaces,
   SharedKeys? sharedKeys,
@@ -414,6 +416,7 @@ Dependencies testDependencies({
               notes,
               paths,
               assets,
+              feedbackEnvelopes ?? FakeFeedbackEnvelopes(),
             ),
             discussions: DiscussionRepository(
               discussionGateway ?? FakeDiscussionGateway(),