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_app/pubspec.yaml

@@ -71,6 +71,10 @@ dev_dependencies:
     sdk: flutter
   # A test drives sign-in through its front, as Serverpod's GitHub sign-in does.
   flutter_web_auth_2: ^5.1.0
+  # The register's own checks, which the device's real cryptography is held
+  # to (register_interop_test.dart).
+  halkyon_base_server:
+    path: ../halkyon_base_server
 
 flutter:
   # Generates HalkyonAppLocalizations from lib/l10n (see l10n.yaml).