feat(app,learn): shared spaces' members on the device

Step 4 (e) of ADR 0044, its first part, for every app's screens.

- halkyon_app: `SharedSpaceGateway` on the module's `sharedSpace`
  endpoint (the spaces, their members and roles, inviting, handing on,
  leaving), refusals as `SpaceRefused`; `SharedKeys.trustIn` gives
  each member's safety code and whether it was compared or their keys
  changed, `markCompared`, and `trustNewKeys`, which pins the keys
  whose code was compared and never those the instance hands next.
  `FakeSharedSpaceServer` keeps the module's rules for tests, in step
  with `FakeSharedKeysServer`, which now knows members' handles.
- Learn's sharing makes its space through it.

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/feature/sharing/studyset_sharing_test.dart

@@ -1,4 +1,6 @@
 import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart'
+    show SharedSpaceGateway, SpaceInfo, SpaceMember, SpaceRole;
 import 'package:halkyon_app/testing.dart'
     show FakeFileServerGateway, MemoryFileCache;
 import 'package:halkyon_editor/halkyon_editor.dart';
@@ -18,7 +20,6 @@ import 'package:halkyon_learn_flutter/src/feature/learning_path/data/path_progre
 import 'package:halkyon_learn_flutter/src/feature/note/data/note_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/pdf_source/data/pdf_source_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/publish/model/publication.dart';
-import 'package:halkyon_learn_flutter/src/feature/sharing/data/shared_space_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/sharing/data/studyset_sharing.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/data/review_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/model/rating.dart';
@@ -303,7 +304,8 @@ class _Spaces() implements SharedSpaceGateway {
   final deleted = <String>[];
 
   @override
-  Future<String> create() async {
+  Future<String> create(String kind) async {
+    expect(kind, 'studyset');
     final space = 'space-${made.length + 1}';
     made.add(space);
     return space;
@@ -311,6 +313,32 @@ class _Spaces() implements SharedSpaceGateway {
 
   @override
   Future<void> delete(String space) async => deleted.add(space);
+
+  @override
+  Future<List<SpaceInfo>> mine() => throw UnimplementedError();
+
+  @override
+  Future<List<SpaceMember>> members(String space) => throw UnimplementedError();
+
+  @override
+  Future<SpaceMember> invite(String space, String handle, SpaceRole role) =>
+      throw UnimplementedError();
+
+  @override
+  Future<void> setRole(String space, String member, SpaceRole role) =>
+      throw UnimplementedError();
+
+  @override
+  Future<void> remove(String space, String member) =>
+      throw UnimplementedError();
+
+  @override
+  Future<void> handOn(String space, String member) =>
+      throw UnimplementedError();
+
+  @override
+  Future<void> leave(String space, {String? nextOwner}) =>
+      throw UnimplementedError();
 }
 
 /// The device's side of a new space: the spaces readied and the studysets