refactor(app,learn): people's files on the device shared in halkyon_app

ADR 0043's recommendation, for Todo's attachments: what Learn had for its
files on the device moves into halkyon_app, unchanged but for its names,
and Learn uses it from there.

- FileCache, DirectoryFileCache (Learn's AssetFileCache): the files by
  their SHA-256 in a folder of the person's, and which wait to be
  uploaded.
- FileServerGateway, ServerpodFileServerGateway (Learn's
  AssetServerGateway): halkyon_base's files endpoint, taking the module's
  caller rather than Learn's client; FileLimitReached, FileRoom.
- Their fakes in halkyon_app's testing: MemoryFileCache,
  FakeFileServerGateway, sha256Of.
- SealedFile: the format every app seals a file in (HLKF, version 1),
  sealing with halkyon_app's ContentCipher; Learn's Privacy seals and
  opens its files with it, for the same context as before, so every file
  sealed so far opens as it did.

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/feature/anki/data/anki_importer_test.dart

@@ -1,6 +1,8 @@
 import 'dart:io';
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/testing.dart'
+    show FakeFileServerGateway, MemoryFileCache;
 import 'package:halkyon_editor/halkyon_editor.dart';
 import 'package:halkyon_learn_flutter/src/core/asset/data/asset_repository.dart';
 import 'package:halkyon_learn_flutter/src/core/work/work_progress.dart';
@@ -22,7 +24,6 @@ import 'package:halkyon_learn_flutter/src/feature/tag/data/tag_repository.dart';
 
 import '../../../support/fake_asset_gateway.dart';
 import '../../../support/fake_asset_sealing.dart';
-import '../../../support/fake_asset_server_gateway.dart';
 import '../../../support/fake_card_gateway.dart';
 import '../../../support/fake_learning_path_gateway.dart';
 import '../../../support/fake_note_gateway.dart';
@@ -30,7 +31,6 @@ import '../../../support/fake_pdf_source_gateway.dart';
 import '../../../support/fake_review_gateway.dart';
 import '../../../support/fake_studyset_gateway.dart';
 import '../../../support/fake_tag_gateway.dart';
-import '../../../support/memory_asset_file_cache.dart';
 import 'anki_package_test.dart' show fixture;
 
 void main() {
@@ -55,8 +55,8 @@ void main() {
     assetRows = FakeAssetGateway();
     assets = AssetRepository(
       assetRows,
-      FakeAssetServerGateway(),
-      MemoryAssetFileCache(),
+      FakeFileServerGateway(),
+      MemoryFileCache(),
       FakeAssetSealing(),
     );
     importer = AnkiImporter(