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/assistant/data/assistant_files_test.dart

@@ -3,15 +3,15 @@ import 'dart:typed_data';
 import 'dart:ui' show Rect;
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/testing.dart'
+    show FakeFileServerGateway, MemoryFileCache;
 import 'package:halkyon_learn_flutter/src/core/asset/data/asset_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/assistant/data/assistant_files.dart';
 import 'package:halkyon_learn_flutter/src/feature/pdf_source/model/pdf_source.dart';
 
 import '../../../support/fake_asset_gateway.dart';
 import '../../../support/fake_asset_sealing.dart';
-import '../../../support/fake_asset_server_gateway.dart';
 import '../../../support/fake_pdf_engine.dart';
-import '../../../support/memory_asset_file_cache.dart';
 
 // What a model is sent of the person's files (ADR 0021, D12): images every
 // provider reads, none larger than the smallest limit, and a PDF's pages
@@ -25,8 +25,8 @@ void main() {
   setUp(() {
     assets = AssetRepository(
       FakeAssetGateway(),
-      FakeAssetServerGateway(),
-      MemoryAssetFileCache(),
+      FakeFileServerGateway(),
+      MemoryFileCache(),
       FakeAssetSealing(),
     );
     engine = FakePdfEngine();