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>

apps/train/halkyon_train_flutter/test/feature/day/day_repository_test.dart

@@ -3,6 +3,8 @@ import 'dart:typed_data';
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_train_client/halkyon_train_client.dart'
+    show Namespace, Uuid;
 import 'package:halkyon_train_flutter/src/core/privacy/train_keys.dart';
 import 'package:halkyon_train_flutter/src/feature/day/data/day_repository.dart';
 import 'package:halkyon_train_flutter/src/feature/day/data/loop_lines.dart';
@@ -69,7 +71,7 @@ void main() {
     final keys = TrainKeys(keyRows, SodiumContentCipher(), () async => account);
     opened.add(keys);
     await keys.refresh();
-    return DayRepository(rows, keys, person: 'alice');
+    return DayRepository(rows, keys);
   }
 
   setUpAll(() async {
@@ -227,6 +229,48 @@ void main() {
     }
   });
 
+  test("no row's ID tells its date: the instance, hashing every date as an "
+      'ID of version 5 would be, finds none of them (D57); another phone of '
+      'the account names the rows alike', () async {
+    final phone = await device(alice);
+    await phone.take(
+      linesOf([
+        summary(tuesday, 9876),
+        heart(tuesday, [1, 4]),
+        sleep,
+      ]),
+    );
+    final ids = {
+      ...rows.dayRows.keys,
+      ...rows.nightRows.keys,
+      ...rows.seriesRows.keys,
+    };
+    expect(ids, hasLength(4));
+
+    String v5(String path) => const Uuid().v5(
+      Namespace.url.value,
+      'https://halkyon.app/train/$path',
+    );
+    final guesses = {
+      for (final person in ['alice', ''])
+        for (final kind in ['day', 'night'])
+          for (final date in [monday, tuesday]) ...[
+            v5('$kind/$person/${dayKey(date)}'),
+            for (var i = 0; i < 8; i++)
+              v5('$kind/$person/${dayKey(date)}/heart/$i'),
+          ],
+    };
+    expect(ids.intersection(guesses), isEmpty);
+    for (final id in ids) {
+      expect(id[14], '8', reason: 'a UUID of version 8, keyed: $id');
+    }
+
+    final other = await device(alice);
+    await other.take(linesOf([summary(tuesday, 9877)]));
+    expect(rows.dayRows.keys, hasLength(1));
+    expect((await other.day(tuesday))!.steps, 9877);
+  });
+
   test(
     "a device without the account's keys sees no day and keeps none",
     () async {

apps/train/halkyon_train_flutter/test/feature/day/days_cubit_test.dart

@@ -38,7 +38,7 @@ void main() {
       () async => FakeAccountCrypto().keysNumbered(1),
     );
     rows = MemoryDayGateway();
-    days = DayRepository(rows, keys, person: 'alice');
+    days = DayRepository(rows, keys);
   });
 
   tearDown(() async {

apps/train/halkyon_train_flutter/test/feature/day/sync_day_gateway_test.dart

@@ -63,7 +63,7 @@ void main() {
       () async => alice,
     );
     await keys.refresh();
-    days = DayRepository(SyncDayGateway(db, sync), keys, person: 'alice');
+    days = DayRepository(SyncDayGateway(db, sync), keys);
   });
 
   tearDown(() async {

apps/train/halkyon_train_flutter/test/feature/loop/polar_loop_test.dart

@@ -101,7 +101,7 @@ void main() {
       person: 'person',
     );
     await profile.save(body);
-    days = DayRepository(MemoryDayGateway(), keys, person: 'person');
+    days = DayRepository(MemoryDayGateway(), keys);
     workouts = WorkoutRepository(MemoryWorkoutGateway(), keys);
     now = DateTime.utc(2026, 10, 28, 9);
     day = today;
@@ -344,7 +344,7 @@ void main() {
       );
       addTearDown(keyless.dispose);
       await keyless.refresh();
-      days = DayRepository(MemoryDayGateway(), keyless, person: 'person');
+      days = DayRepository(MemoryDayGateway(), keyless);
       platform.days.add([summary('2026-10-27', 9876)]);
       final loop = await setUpBand();
       await settled(loop);