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/profile/profile_test.dart

@@ -214,13 +214,58 @@ void main() {
         expect((renamed.handle, renamed.handleFixed), ('Alice_JP', true));
       });
 
-      test('removing a profile frees its handle', () async {
-        await save(alice, 'alice');
+      test('removing a profile makes it private: its handle stays its '
+          "person's, what was filled in goes, and others find it as a handle "
+          'nobody has (ADR 0020, D27)', () async {
+        await save(
+          alice,
+          'Alice',
+          displayName: 'Alice',
+          bio: 'Learning Japanese.',
+          links: ['https://halkyon.app'],
+        );
+        expect(
+          (await endpoints.profiles.byHandle(bob, 'alice')).handle,
+          'Alice',
+        );
 
-        await endpoints.profile.remove(alice);
+        final removed = (await endpoints.profile.remove(alice))!;
 
-        expect(await endpoints.profile.mine(alice), isNull);
-        expect((await save(bob, 'alice')).handle, 'alice');
+        expect(
+          (
+            removed.handle,
+            removed.displayName,
+            removed.bio,
+            removed.avatarUrl,
+            removed.handleFixed,
+          ),
+          ('Alice', '', '', null, false),
+        );
+        expect(removed.links, isEmpty);
+        expect(removed.verifiedLinks ?? const <String>[], isEmpty);
+        expect(
+          (await endpoints.profile.mine(alice))!.toJson(),
+          removed.toJson(),
+        );
+        await expectLater(
+          endpoints.profiles.byHandle(bob, 'alice'),
+          throwsA(isA<ProfileNotFoundException>()),
+        );
+        await expectLater(
+          save(bob, 'alice'),
+          refused(ProfileRefusal.handleTaken),
+        );
+        // Filled in again, it is public again.
+        await save(alice, 'Alice', displayName: 'Alice');
+        expect(
+          (await endpoints.profiles.byHandle(bob, 'alice')).displayName,
+          'Alice',
+        );
+      });
+
+      test('removing nothing gives nothing: without a profile', () async {
+        expect(await endpoints.profile.remove(bob), isNull);
+        expect(await endpoints.profile.mine(bob), isNull);
       });
 
       test('a profile is found by its handle only once its person goes '