fix(auth)!: proofs are bound to the instance the device addresses

Nothing tied a sign-in's proof to the instance it was made for: an
instance a person was lured into adding could pass A, M1 and the code on
to the real register and get a session there, and pass a phrase's proof
on with a password of its own in place of the person's (the second
reading's M6). SRP's identity is now a label, the challenge's ID and
the host the device addressed (passwordIdentity, in halkyon_base_client
and halkyon_base_server alike), and the phrase's proof signs that host,
what it is for (an authenticator's reset or a new password) and the new
password's digest (key proof v2). Every proving call carries the host;
in production it must be the server's apiServer.publicHost (otherwise
otherInstance, which the app shows as an instance it can't trust), in
development any; a member forwards its own. The tests prove that a
relayed sign-in, a proof for another host or purpose, and a swapped
password are refused, nothing changing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/test/integration/register/register_test.dart

@@ -555,6 +555,7 @@ void main() {
               clientValue: answer.clientValue,
               clientProof: answer.clientProof,
               totpCode: codeOf(enrolment, at: now),
+              host: testHost,
             );
             expect(proof.account, annaOnTodo);
             expect(answer.verifiesServer(proof.serverProof), isTrue);
@@ -572,6 +573,7 @@ void main() {
                 clientValue: againAnswer.clientValue,
                 clientProof: againAnswer.clientProof,
                 totpCode: codeOf(enrolment, at: now),
+                host: testHost,
               ),
               refused(PasswordRefusal.wrongCredentials),
             );
@@ -596,13 +598,18 @@ void main() {
               device.recoveryKeys.toJson(),
             );
             now = now.add(totpPeriod);
+            final newPassword = device.newPassword();
             expect(
               await remote.resetPasswordWithPhrase(
                 session,
                 challenge: keyChallenge.challenge,
-                signature: await device.sign(keyChallenge),
-                password: device.newPassword(),
+                signature: await device.sign(
+                  keyChallenge,
+                  password: newPassword,
+                ),
+                password: newPassword,
                 totpCode: codeOf(enrolment, at: now),
+                host: testHost,
               ),
               (account: annaOnTodo, endedBefore: now),
             );
@@ -630,6 +637,7 @@ void main() {
               language: 'en',
               request: resetRequest,
               code: mailbox.codeFor('anna@example.org'),
+              host: testHost,
             );
             expect(pending.takesEffectAt, now.add(resetDelay));
 
@@ -727,6 +735,7 @@ void main() {
                 totpCode: codeOf(enrolment),
                 password: device.newPassword(),
                 keepSession: annasPhone,
+                host: testHost,
               );
               expect(members.sessionsTold.single, (
                 _todo,
@@ -757,6 +766,7 @@ void main() {
                   recoveryProofKey: ByteData.sublistView(
                     device.recoveryProofKey,
                   ),
+                  host: testHost,
                 );
               }
 
@@ -861,6 +871,7 @@ void main() {
               clientProof: answer.clientProof,
               request: await code(),
               code: mailbox.codeFor('carla@example.org'),
+              host: testHost,
             );
             expect(proof.account, carla);
             expect(proof.needsAuthenticator, isTrue);