perf: counts worked out once and kept until something changes

On a copy of a real library (8,067 cards, 20,907 reviews, 20
studysets), every open list read every card (0.8 s), every review
(0.25 s) and replayed schedules after every change, each on its own:
the library, every studyset open on the stack and its topics.

- CardRepository keeps each studyset's cards and every card's stamp
  until a card changes; a read begun before a change keeps nothing.
- ReviewRepository keeps the whole log once many cards' reviews are
  asked for, updated by this device's writes and read anew after a
  sync brings others'.
- StudysetProgressRepository works out every studyset's progress once
  for every list and hands the last to whoever follows it next;
  StudysetListBloc reads it.

A full recount now takes about 65 ms. Home no longer rereads every
revision after each rating.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/support/fake_review_gateway.dart

@@ -20,20 +20,23 @@ class FakeReviewGateway() implements ReviewGateway {
   Future<List<ReviewRow>> allReviews() async => [...rows];
 
   @override
-  Future<void> insert(Iterable<ReviewRow> reviews) async {
+  Future<List<ReviewRow>> insert(Iterable<ReviewRow> reviews) async {
     if (failWrites case final error?) {
       throw error;
     }
-    for (final review in reviews) {
-      rows.add((
-        id: 'review-${(_nextId++).toString().padLeft(4, '0')}',
-        cardId: review.cardId,
-        rating: review.rating,
-        reviewedAt: review.reviewedAt,
-        durationMs: review.durationMs,
-        mode: review.mode,
-      ));
-    }
+    final stored = [
+      for (final review in reviews)
+        (
+          id: 'review-${(_nextId++).toString().padLeft(4, '0')}',
+          cardId: review.cardId,
+          rating: review.rating,
+          reviewedAt: review.reviewedAt,
+          durationMs: review.durationMs,
+          mode: review.mode,
+        ),
+    ];
+    rows.addAll(stored);
+    return stored;
   }
 
   @override