docs(adr): signing up in one call, the password's own proof key, changes with a fresh SimpleLogin sign-in (ADR 0020)

D18 to D20, the developer's, before step 3: the device makes the handle,
the keys and the recovery phrase before one `simpleLogin.signUp`, so no
account is ever unfinished; a third subkey of the password's Argon2id
seeds an Ed25519 key the register checks, and a new device gets the
wrapped keys only after proving the password with it; a new password, a
new phrase and deleting the account take the password's proof and a
SimpleLogin sign-in of the last ten minutes. Step 2's deviations from the
proposal are noted as accepted.

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

@@ -1,7 +1,7 @@
 # ADR 0020: Signing in with SimpleLogin only
 
-- **Status:** decided 23 Oct 2026, by the developer (D1–D17 below), the *Proposal*
-  with it (D15).
+- **Status:** decided 23 Oct 2026, by the developer (D1–D20 below), the *Proposal*
+  with it (D15), as D18 to D20 change it.
 - **Built (23 Oct 2026), step 1:** the forwarding page (3),
   `/simplelogin-callback` on the register's web server (halkyon_base's
   `addSimpleLoginCallbackRoute`, Caddy forwarding the path), ahead of the sign-in, so
@@ -28,7 +28,8 @@
   them; GitHub links nobody more where SimpleLogin is offered, as where passwords are
   (ADR 0017); `SIGN_IN_WAYS` counts on the register, which members ask, and stops any
   server with a name it doesn't know as it starts; sessions name the method
-  `simplelogin`. Someone new gets no account yet: their identity waits (D16), with a
+  `simplelogin` (these deviations accepted by the developer, 23 Oct). Someone new gets
+  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.
 - **Builds on:** ADR 0010 (email sign-in), ADR 0011 (profile pages), ADR 0016 (the
@@ -130,6 +131,25 @@ against app.simplelogin.io on 23 Oct:
   cut to 30 characters, and where the handle is taken (whatever its case) or kept by the
   instance, a number from 2 to 99 after it. None where nothing of the name is left (a
   name in another script): the person types one.
+- **D18 Signing up in one call** (23 Oct, night, recommended, over the proposal's two
+  steps): the device takes the handle, makes the keys, asks the password twice and shows
+  the recovery phrase (three of its words typed back), then `simpleLogin.signUp` makes
+  the account and its session together, the newcomer's identity taken once. No account is
+  ever unfinished, so D11's deletion after a day has nothing left to delete.
+- **D19 The password proven with a proof key of its own, the keys only after it** (23
+  Oct, night, recommended, over the keys coming with the session, as Bitwarden's SSO
+  has it, and over keeping SRP): a third subkey of the password's Argon2id seeds an
+  Ed25519 key, whose public half the register keeps; the device signs a one-time nonce
+  with it as it signs with the recovery phrase's (ADR 0017, the same challenges and
+  message). A new device gets the wrapped keys once it has proven the password, so
+  someone who took over a SimpleLogin account can only guess the password online,
+  against the limits, never offline. SimpleLogin accounts keep no SRP verifier.
+- **D20 Changes take the password and a fresh SimpleLogin sign-in** (23 Oct, night,
+  recommended, over the password's proof alone): a new password, a new recovery phrase
+  and deleting the account take the password's proof (the phrase's, for a lost
+  password) and a SimpleLogin sign-in of the last ten minutes, the two factors of ADR
+  0017's third reading with SimpleLogin's and Proton's own second factor in TOTP's
+  place.
 - **Also (23 Oct):** the developer's own account moves by adding SimpleLogin to it while
   signed in with GitHub; the instance's SimpleLogin app goes up for approval once
   halkyon.app/learn exists, before learn.halkyon.eu switches.