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>

docs/adr/0020-sign-in-with-simplelogin.md

@@ -32,6 +32,14 @@
   no account yet: their identity waits (D16), with a
   handle suggested (D17), for the sign-up of step 3. halkyon_oidc's `testing.dart` gives
   hosts a fake SimpleLogin signing real ID tokens, which the module's tests use too.
+- **Built (23 Oct 2026), step 3 so far:** the password's own proof key (`halkyon_keys`'
+  subkey 3 of the master key, D19), which `account_password` keeps in place of SRP's
+  verifier for an account that signs in with SimpleLogin (the verifier becomes nullable:
+  a migration that drops nothing); `simpleLogin.signUp` in one call (D18) and
+  `simpleLogin.checkHandle`, both for someone new's identity alone; `keys.begin` and
+  `keys.unlock`, the proof counted as ADR 0017's; every sign-in into an account kept ten
+  minutes under its attempt as D20's confirmation. The changes (first keys, a new or
+  lost password, a new phrase, the deletion) and the mending come next.
 - **Builds on:** ADR 0010 (email sign-in), ADR 0011 (profile pages), ADR 0016 (the
   register), ADR 0017 (a password, TOTP and the keys).
 - **Changes:** ADR 0017's way in on an instance that offers SimpleLogin (its keys, their