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_keys/lib/halkyon_keys.dart

@@ -30,4 +30,5 @@ export 'src/recovery_phrase.dart'
         recoveryPhraseOf,
         recoveryProofKeysOf,
         recoveryProofPublicKeyOf;
+export 'src/row_ids.dart' show keyedRowId;
 export 'src/sealed_content.dart' show SealedContent, openContent, sealContent;

packages/halkyon_keys/lib/src/row_ids.dart

@@ -0,0 +1,41 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:halkyon_keys/src/account_keys.dart';
+import 'package:sodium/sodium_sumo.dart';
+
+/// Row IDs keyed by the account (Halkyon Train's ADR 0030, D57): a row that
+/// every device of the account must name alike without asking each other
+/// (a day's, by its date) takes its ID from its name under a key only the
+/// account holds, so that two devices offline still write the same row, and
+/// the instance, which sees the ID, learns nothing of the name. A name
+/// hashed in the open (a UUID of version 5) would tell it: the instance can
+/// hash every date itself.
+
+/// What the ID's hash binds itself to before the name: a label naming the
+/// format, so that no other use of the key meets it.
+final _rowIdLabel = utf8.encode('halkyon row id v1');
+
+/// The ID of the row named [name] for the account of [keys]: BLAKE2b keyed
+/// with the account's X25519 secret key (16 bytes, over the label and the
+/// name), as a UUID of version 8 (RFC 9562, for IDs of one's own making).
+/// The same on every device of the account, another for any other name or
+/// account, and nothing of [name] read from it without the key. Throws an
+/// [ArgumentError] for an empty name.
+String keyedRowId(SodiumSumo sodium, AccountKeys keys, String name) {
+  if (name.isEmpty) {
+    throw ArgumentError.value(name, 'name', 'A row ID needs a name');
+  }
+  final hash = sodium.crypto.genericHash(
+    message: Uint8List.fromList([..._rowIdLabel, 0, ...utf8.encode(name)]),
+    key: keys.encryption.secretKey,
+    outLen: 16,
+  );
+  hash[6] = hash[6] & 0x0f | 0x80;
+  hash[8] = hash[8] & 0x3f | 0x80;
+  final hex = [
+    for (final byte in hash) 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)}';
+}