feat(explore): take a later release into a copy, at a tap

A copy's top studyset says when a newer release of what it came from is
out, and what it would bring: changed, new and withdrawn items, counted
against the release the copy last took. Update takes it, never by
itself (ADR 0005, rule 4): cards, notes and steps take the release as a
revision on the one where they last took a release and merge per side,
as a dictionary's corrections do, so what the learner changed stays and
a side both changed is a conflict to settle; studysets, paths and PDFs
take each field the learner left as it was. New items come in where
they belong, unless the learner deleted that part; what the author
withdrew stays, marked, and the card browser filters for it; what the
learner deleted stays deleted; progress is never touched.

The instance gives out any release by its version, for the app to
compare the two. UpdateTaken moves into the shared revision merge, now
that notes and steps take updates too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/feature/card/data/card_repository_test.dart

@@ -3,6 +3,7 @@ import 'dart:typed_data';
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:halkyon_learn_flutter/src/core/content/payload.dart';
+import 'package:halkyon_learn_flutter/src/core/content/revision_merge.dart';
 import 'package:halkyon_learn_flutter/src/core/content/rich_content.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/card_payload.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/data/card_repository.dart';