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>

packages/halkyon_app/test/shared_space/shared_keys_test.dart

@@ -219,4 +219,43 @@ void main() {
 
     expect(await bob.openMine(space), openedThese({}, refused: {'k1'}));
   });
+
+  test("a member's safety code is the same on both sides, and is marked "
+      'compared once the person says so (D32)', () async {
+    final seen = (await alice.trustIn(space)).single;
+    expect(seen.person, 'bob');
+    expect(seen.changed, isFalse);
+    expect(seen.compared, isFalse);
+    expect(seen.code, (await bob.trustIn(space)).single.code);
+
+    await alice.markCompared('bob');
+    expect((await alice.trustIn(space)).single.compared, isTrue);
+  });
+
+  test('keys that changed are flagged with the code of the new ones, and '
+      'only the keys whose code was compared are trusted anew', () async {
+    await alice.trustIn(space);
+    await alice.markCompared('bob');
+    final mallorys = keysOf(malloryAccount);
+    final eves = keysOf((await crypto.newAccount('eve password')).keys);
+    server.handOutKeys('bob', mallorys);
+
+    final changed = (await alice.trustIn(space)).single;
+    expect(changed.changed, isTrue);
+    expect(changed.compared, isFalse);
+    expect(changed.code, aliceCipher.safetyCodeWith(mallorys));
+
+    // The instance hands out other keys again before the person trusts the
+    // ones they compared: those compared are pinned, and these differ.
+    server.handOutKeys('bob', eves);
+    await alice.trustNewKeys('bob', changed.keys);
+    final after = (await alice.trustIn(space)).single;
+    expect(after.changed, isTrue);
+    expect(after.code, aliceCipher.safetyCodeWith(eves));
+
+    server.handOutKeys('bob', mallorys);
+    final trusted = (await alice.trustIn(space)).single;
+    expect(trusted.changed, isFalse);
+    expect(trusted.compared, isTrue);
+  });
 }