feat(base,app,keys): keys of shared spaces, sealed to each member

Step 3 of ADR 0044 (D3, D18, D19). A shared space's data keys are
sealed on a device to each member's public keys; the instance keeps
what devices sealed and never opens it.

- halkyon_keys: the safety code of two accounts' public keys
  (`safetyCode`), 60 digits, the same on both sides.
- halkyon_base: the register hands out accounts' public keys
  (`public-keys-of`, local and remote); the table `shared_space_key`
  and the endpoint `sharedSpaceKeys` (memberKeys, keysNeeded, seal,
  mine). Only the Owner and Editors seal, the first key sealed to a
  member stays, and a member's keys go with their membership. Every
  host's migration creates the table.
- halkyon_app: `ContentCipher` seals a key to another account and
  opens one sealed by another; `SharedKeys` shares a space's key with
  its members, seals keys to newcomers and opens the person's own,
  holding each member to the public keys `KeyPins` pinned on first
  sight (`key_pins.json` in the person's folder), a change refused
  until trusted anew.

Learn takes them into its `Privacy` in step 4.

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/safety_code_test.dart

@@ -0,0 +1,64 @@
+import 'package:halkyon_keys/halkyon_keys.dart';
+import 'package:sodium/sodium_sumo.dart';
+import 'package:test/test.dart';
+
+Future<void> main() async {
+  final sodium = await SodiumSumoInit.init();
+
+  PublicKeys keysOf(AccountKeys account) => (
+    encryption: account.encryptionPublicKey,
+    signing: account.signingPublicKey,
+  );
+
+  group('safety codes (ADR 0044, D18)', () {
+    final alice = AccountKeys.generate(sodium);
+    final bob = AccountKeys.generate(sodium);
+
+    test('both people see the same 60 digits, in twelve groups of five', () {
+      final code = safetyCode(sodium, keysOf(alice), keysOf(bob));
+      expect(code, safetyCode(sodium, keysOf(bob), keysOf(alice)));
+      expect(code, matches(RegExp(r'^\d{5}( \d{5}){11}$')));
+    });
+
+    test('another key on either side, of either kind, gives another '
+        'code', () {
+      final code = safetyCode(sodium, keysOf(alice), keysOf(bob));
+      final mallory = AccountKeys.generate(sodium);
+      expect(safetyCode(sodium, keysOf(alice), keysOf(mallory)), isNot(code));
+      expect(
+        safetyCode(sodium, keysOf(alice), (
+          encryption: mallory.encryptionPublicKey,
+          signing: bob.signingPublicKey,
+        )),
+        isNot(code),
+      );
+      expect(
+        safetyCode(sodium, keysOf(alice), (
+          encryption: bob.encryptionPublicKey,
+          signing: mallory.signingPublicKey,
+        )),
+        isNot(code),
+      );
+    });
+
+    test("half of the code is each person's own, whoever they compare "
+        'with', () {
+      final carol = AccountKeys.generate(sodium);
+      final withBob = safetyCode(
+        sodium,
+        keysOf(alice),
+        keysOf(bob),
+      ).replaceAll(' ', '');
+      final withCarol = safetyCode(
+        sodium,
+        keysOf(alice),
+        keysOf(carol),
+      ).replaceAll(' ', '');
+      final halves = {
+        withBob.substring(0, 30),
+        withBob.substring(30),
+      }.intersection({withCarol.substring(0, 30), withCarol.substring(30)});
+      expect(halves, hasLength(1));
+    });
+  });
+}