feat(base)!: a handle at password sign-up (ADR 0020, D26, D28–D30)

Every new account gets a handle, and an account without one gets its
first as it sets a password. The handle is private until its person
goes public (D5).

- `passwordSignIn.signUp` takes a handle. `passwordSignIn.checkHandle`
  says whether a sign-up could take one, only while that sign-up's
  mailed code waits (`codeExpired` otherwise), counted per code request
  (30 an hour) and per network (60) (D29). Members ask the register.
- `password.set` takes the first handle of an account without one
  (D28); `SignInWays.handle` tells the app whether there is one.
- Each checks the handle before the code is used, so a refusal leaves
  the code for another try. An existing account at the address keeps
  its own handle where it has one, and the typed handle is still
  checked, so the answer says nothing about the address.
- `first_handle.dart` holds what SimpleLogin, the keys and passwords
  share: `refusalOfNewHandle`, `insertFirstProfile`, `hasHandle`.
- halkyon_app: one `HandleField` with its `HandleCheckCubit` (asked
  300 ms after typing pauses, only about the handle typed last; the
  instance's suggestion is free without asking) (D30). The password
  setup asks for the handle between the phrase check and the code, and
  a refusal at the code sends back to it. SimpleLogin's sign-up and
  keys steps use the same field and now check as it is typed.
- The module's README and ADR 0020 (D29, D30, the built state) 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/auth/widget/password_set_up_test.dart

@@ -48,7 +48,7 @@ void main() {
   }
 
   testWidgets('an account without a password sets one up: the address, the '
-      'phrase checked, the code, the authenticator', (
+      'phrase checked, the handle it lacks, the code, the authenticator', (
     tester,
   ) async {
     await openProfile(tester);
@@ -75,6 +75,11 @@ void main() {
       );
     }
     await next(tester, 'Continue');
+    await tester.enterText(field('Handle'), 'anna');
+    await tester.pump(HandleCheckCubit.pause);
+    await tester.pumpAndSettle();
+    expect(find.text('@anna is free'), findsOneWidget);
+    await next(tester, 'Continue');
     await tester.enterText(
       field('Code from the email'),
       passwords.codeFor('anna@example.org'),
@@ -91,6 +96,7 @@ void main() {
     expect(find.text('Your progress is kept with your account.'), findsOne);
     expect(keys.kept.keys, [FakeAuthGateway.person.authUserId]);
     expect(passwords.accounts['anna@example.org']!.finished, isTrue);
+    expect(passwords.handles[FakeAuthGateway.person.authUserId], 'anna');
     await tester.dragUntilVisible(
       find.text('Change password'),
       find.byType(ListView).last,