test(app): the device's real cryptography meets the register's checks

The register's tests make the device's side from made-up bytes, and the
app's from fakes; nothing held what the device's real cryptography
(libsodium, on isolates) makes to what the register takes. Now
register_interop_test.dart does: a new account's setup passes
isPasswordSetup, the device's SRP answer checks with the register's
srpVerify and its M2 with the device, bound to the instance and the
purpose (another instance's or a change's identity refused), the keys
open as made, a new password passes isNewPassword, and the phrase's proof
for it checks against the phrase's own key, never the account's, with
the device's framing of the message the register's. halkyon_base_server
exports the checks it uses; halkyon_app takes it as a dev dependency.

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/support/password_device.dart

@@ -5,7 +5,6 @@ import 'package:clock/clock.dart';
 import 'package:cryptography/cryptography.dart';
 import 'package:halkyon_base_client/srp.dart' as srp;
 import 'package:halkyon_base_server/base.dart';
-import 'package:halkyon_base_server/src/srp/srp_math.dart';
 
 /// The base URL devices address the tests' server by: its `apiServer`'s
 /// public scheme, host and port (config/test.yaml), which SRP's identity