fix(keys): a device refuses Argon2id costs beyond the register's ceiling

PasswordKdf.fromJson refused costs below the floor but took any above it,
so an instance could ask a device for 100 GiB and crash the app at every
sign-in. It now refuses more than the register's own ceiling (16 passes,
1 GiB; KeyRefusal.excessivePasswordKdf), and the app reads either refusal,
before deriving or sending anything, as PasswordProblem.unsafeCosts with
a text of its own (before, costs below the floor read as keys missing).

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_keys/test/password_keys_test.dart

@@ -103,6 +103,34 @@ Future<void> main() async {
       expect(read.memLimit, PasswordKdf.minimum.memLimit);
     });
 
+    test("the ceiling itself is read: the register's own, 16 passes and "
+        '1 GiB', () {
+      final read = PasswordKdf.fromJson(PasswordKdf.maximum.toJson());
+      expect(read.opsLimit, 16);
+      expect(read.memLimit, 1024 * 1024 * 1024);
+    });
+
+    test('above the ceiling are refused: a register asking for more would '
+        'make the device run out of memory, or time', () {
+      for (final json in <Map<String, Object?>>[
+        {'algorithm': 'argon2id13', 'opsLimit': 17, 'memLimit': 64 << 20},
+        {'algorithm': 'argon2id13', 'opsLimit': 3, 'memLimit': (1 << 30) + 1},
+        {'algorithm': 'argon2id13', 'opsLimit': 3, 'memLimit': 100 << 30},
+      ]) {
+        expect(
+          () => PasswordKdf.fromJson(json),
+          throwsA(
+            isA<KeyRefused>().having(
+              (refused) => refused.reason,
+              'reason',
+              KeyRefusal.excessivePasswordKdf,
+            ),
+          ),
+          reason: '$json',
+        );
+      }
+    });
+
     test('round-trip', () {
       const raised = PasswordKdf(opsLimit: 4, memLimit: 128 * 1024 * 1024);
       final json = raised.toJson();