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/support/fake_profile_gateway.dart

@@ -145,14 +145,23 @@ class FakeProfileGateway({
   }
 
   @override
-  Future<void> remove() async {
+  Future<PublicProfile?> remove() async {
     if (fail case final error?) {
       throw error;
     }
-    if (profile?.handleFixed ?? false) {
+    final current = profile;
+    if (current == null) {
+      return null;
+    }
+    if (current.handleFixed) {
       throw ProfileRefused(ProfileProblem.published);
     }
-    profile = null;
+    // Private again, the handle kept (ADR 0020, D27).
+    return profile = PublicProfile(
+      handle: current.handle,
+      address: current.address,
+      pageUrl: current.pageUrl,
+    );
   }
 
   /// The links whose pages link back to the profile page.