feat(base): SimpleLogin accounts' keys without SRP, signing up in one call (ADR 0020, step 3)

An account that signs in with SimpleLogin keeps its keys under a password
that signs nobody in (D19): `account_password` keeps the password's own
proof key (Ed25519, `passwordProofKey`) in place of SRP's verifier, which
becomes nullable. The migration adds the column and drops the NOT NULL,
nothing else.

- `simpleLogin.signUp(attempt, handle, setup)`: someone new whose identity
  waits under the attempt gets, in one transaction, an account
  (identified), the link, a profile with the handle and the keys; then a
  session (D18). Refused, the identity still waiting, for a setup out of
  range and a handle refused as saving a profile refuses it.
- `simpleLogin.checkHandle(attempt, handle)`: whether someone new could
  take a handle, at most 30 an hour per attempt and 60 per network.
- `simpleLogin.complete` says, for an account, whether it has its keys and
  its handle, and keeps the sign-in ten minutes under its attempt, the
  account as its caller: D20's confirmation for the changes to come.
- `keys.begin`, `keys.unlock`: the keys for a signed-in device once the
  password is proven with its proof key, for a nonce, bound to the base
  URL the device addressed; wrong proofs counted per session as ADR
  0017's.
- An authenticator is enrolled only for a password that signs in.
- Members ask the register for all of it; `RemoteRegister` and the
  instance API carry the four new operations.

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_client/lib/migrations/migration_registry.dart

@@ -29,6 +29,7 @@ part '20261021110159867-second-factor-paths/migration.dart';
 part '20261021134803749-reset-hold-outs/migration.dart';
 part '20261021184601197-private-studysets/migration.dart';
 part '20261023181810112-oidc-module/migration.dart';
+part '20261023192913178-simplelogin-keys/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -60,6 +61,7 @@ class MigrationRegistry {
     _Migration20261021134803749_reset_hold_outs(),
     _Migration20261021184601197_private_studysets(),
     _Migration20261023181810112_oidc_module(),
+    _Migration20261023192913178_simplelogin_keys(),
   ];
 
   /// List of all client-side database migration versions.