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>

packages/halkyon_base_server/test/integration/register/member_test.dart

@@ -181,8 +181,23 @@ final class _Register implements AccountRegister {
       _answer('removeAvatar', () => profiles[person]!);
 
   @override
-  Future<void> removeProfile(Session session, UuidValue person) =>
-      _answer('removeProfile', () => profiles.remove(person));
+  Future<Profile?> removeProfile(Session session, UuidValue person) =>
+      _answer('removeProfile', () {
+        final profile = profiles[person];
+        if (profile == null) {
+          return null;
+        }
+        // Private again, the handle kept (ADR 0020, D27).
+        return profiles[person] = Profile(
+          handle: profile.handle,
+          address: profile.address,
+          displayName: '',
+          bio: '',
+          links: const [],
+          pageUrl: profile.pageUrl,
+          handleFixed: profile.handleFixed,
+        );
+      });
 
   @override
   Future<void> fixHandle(Session session, UuidValue person) =>
@@ -1710,6 +1725,17 @@ void main() {
           expect(await PublicProfile.db.count(session), 0);
         });
 
+        test('are removed at the register, which says what is left: the '
+            'handle (ADR 0020, D27)', () async {
+          await endpoints.profile.save(asBen(), 'Ben', 'Ben', '', const []);
+
+          final removed = (await endpoints.profile.remove(asBen()))!;
+
+          expect((removed.handle, removed.displayName), ('Ben', ''));
+          expect(register.asked.last, 'removeProfile');
+          expect(register.profiles[ben]!.toJson(), removed.toJson());
+        });
+
         test('have their handles checked at the register, the device '
             'network passed on to be counted there', () async {
           register.handleRefusal = ProfileRefusal.tooManyHandleChecks;