feat(explore): the device tells the instance whether to count the person, and what their copies come from (ADR 0019, D3)

Count me stays in the person's settings, which follow them to every device
and are now encrypted; after each sync the device mirrors it to the
instance (`learningCount.setCounted`) and, only while it is on, reports
which publications their copies come from, topics with them
(`reportCopies`). Taken back, the instance forgets the copies. Only people
with an identity report, as only they are counted.

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/learn/halkyon_learn_flutter/test/support/fake_learning_count_gateway.dart

@@ -0,0 +1,29 @@
+import 'package:halkyon_learn_flutter/src/feature/explore/data/learning_count_gateway.dart';
+
+/// [LearningCountGateway] in memory: what the instance would keep.
+class FakeLearningCountGateway() implements LearningCountGateway {
+  /// Whether the instance counts the person.
+  bool isCounted = false;
+
+  /// The copies last reported, kept only while counted, as the instance
+  /// keeps them.
+  Map<String, Set<String>>? reported;
+
+  @override
+  Future<bool> counted() async => isCounted;
+
+  @override
+  Future<void> setCounted({required bool counted}) async {
+    isCounted = counted;
+    if (!counted) {
+      reported = null;
+    }
+  }
+
+  @override
+  Future<void> reportCopies(Map<String, Set<String>> copies) async {
+    if (isCounted) {
+      reported = copies;
+    }
+  }
+}