feat(base)!: removing a profile makes it private (ADR 0020, D27)

Every account keeps a handle now, so removing a public profile no
longer deletes it and frees the handle: it takes away what the profile
shows and leaves it private, as a profile is until its person goes
public (D5).

- `profile.remove` and the register's `removeProfile` (members ask the
  register) empty the display name, bio, links with their checks and
  the avatar, keep the row with its handle, and return what is left,
  null for none. `/@handle` and `profiles.byHandle` then answer as for
  a handle nobody has. Once the person has published, removing is
  refused as before.
- Learn: the profile form starts afresh from what is left, "Remove
  profile" shows only while something is filled in, and the dialog says
  the handle stays.
- The module's README and ADR 0020 amended.

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/profile/public_profile_test.dart

@@ -212,25 +212,56 @@ void main() {
     expect(find.text('Remove profile'), findsNothing);
   });
 
-  testWidgets('removing a profile asks first, then frees its handle', (
-    tester,
-  ) async {
+  testWidgets('removing a profile asks first, then makes it private: the '
+      'handle stays, what was filled in goes (ADR 0020, D27)', (tester) async {
     final profiles = FakeProfileGateway(
       profile: const PublicProfile(
         handle: 'raisondetredev',
         address: 'raisondetredev@halkyon.eu',
+        displayName: 'Raison',
+        bio: 'Learning Japanese.',
+        links: ['https://halkyon.app'],
       ),
     );
     await showProfile(tester, profiles);
+    expect(find.widgetWithText(TextField, 'Raison'), findsOneWidget);
 
     await tester.tap(find.text('Remove profile'));
     await tester.pumpAndSettle();
     expect(find.text('Remove your public profile?'), findsOneWidget);
+    expect(find.textContaining('Your handle stays yours'), findsOneWidget);
     await tester.tap(find.text('Remove profile').last);
     await tester.pumpAndSettle();
 
     expect(find.text('Profile removed.'), findsOneWidget);
-    expect(profiles.profile, isNull);
+    expect(profiles.profile?.handle, 'raisondetredev');
+    expect(profiles.profile?.displayName, '');
+    expect(find.text('@raisondetredev'), findsOneWidget);
+    // The form starts afresh from what is left.
+    expect(find.widgetWithText(TextField, 'Raison'), findsNothing);
+    expect(find.widgetWithText(TextField, 'Learning Japanese.'), findsNothing);
+    expect(
+      find.widgetWithText(TextField, 'https://halkyon.app'),
+      findsNothing,
+    );
+    // Nothing is left to remove.
+    expect(find.text('Remove profile'), findsNothing);
+  });
+
+  testWidgets('offers no removing while nothing is filled in: the handle '
+      'alone is private already', (tester) async {
+    await showProfile(
+      tester,
+      FakeProfileGateway(
+        profile: const PublicProfile(
+          handle: 'raisondetredev',
+          address: 'raisondetredev@halkyon.eu',
+        ),
+      ),
+    );
+
+    expect(find.text('@raisondetredev'), findsOneWidget);
+    expect(find.text('Remove profile'), findsNothing);
   });
 
   testWidgets('a saved profile takes a picture as its avatar, and gives it '