fix(train): no row's ID tells the instance a date: the days' IDs are keyed by the account (ADR 0030, D57)

A day's, night's and heart chunk's ID was a UUID of version 5 of the
person and the date, which the instance can hash for every date itself:
each ID told it which days the band was worn. halkyon_keys' keyedRowId
hashes the name under the account's X25519 secret key (BLAKE2b, a UUID of
version 8), the same on every device of the account and unreadable
without its keys; ContentCipher.rowId and TrainKeys.rowId hand it on.

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/lib/src/privacy/content_cipher.dart

@@ -69,6 +69,12 @@ abstract interface class ContentCipher() {
   /// [unlock].
   bool verify(List<int> message, List<int> signature);
 
+  /// The ID of the row named [name], keyed by the account (halkyon_keys'
+  /// `keyedRowId`, Halkyon Train's ADR 0030, D57): the same on each of its
+  /// devices, and nothing of [name] read from it by the instance. Throws a
+  /// [StateError] before [unlock].
+  String rowId(String name);
+
   /// Wipes every key kept open here.
   void dispose();
 }
@@ -213,6 +219,12 @@ class SodiumContentCipher() implements ContentCipher {
     );
   }
 
+  @override
+  String rowId(String name) {
+    final (sodium, account) = _unlocked();
+    return keyedRowId(sodium, account, name);
+  }
+
   @override
   void dispose() {
     for (final key in _keys.values) {

packages/halkyon_app/lib/src/testing/fake_content_cipher.dart

@@ -96,6 +96,25 @@ class FakeContentCipher() implements ContentCipher {
       signature.length == 64 &&
       _same(Uint8List.fromList(signature), sign(message));
 
+  /// A UUID of version 8 made from the account and [name], as the real
+  /// one's form; but nothing here keys it.
+  @override
+  String rowId(String name) {
+    final account = _unlocked();
+    final bytes = Uint8List(16);
+    final tag = utf8.encode('row|$account|$name');
+    for (var i = 0; i < tag.length; i++) {
+      bytes[i % 16] = (bytes[i % 16] * 31 + tag[i]) & 0xff;
+    }
+    bytes[6] = bytes[6] & 0x0f | 0x80;
+    bytes[8] = bytes[8] & 0x3f | 0x80;
+    final hex = [
+      for (final byte in bytes) byte.toRadixString(16).padLeft(2, '0'),
+    ].join();
+    return '${hex.substring(0, 8)}-${hex.substring(8, 12)}-'
+        '${hex.substring(12, 16)}-${hex.substring(16, 20)}-${hex.substring(20)}';
+  }
+
   @override
   void dispose() => opened.clear();