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/card_proposals_sheet_test.dart

@@ -20,13 +20,11 @@ import 'package:material_ui/material_ui.dart' hide Card;
 
 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_note_gateway.dart';
 import '../../support/fake_pdf_engine.dart';
 import '../../support/fake_pdf_source_gateway.dart';
 import '../../support/fake_studyset_gateway.dart';
-import '../../support/memory_asset_file_cache.dart';
 import '../../support/pump_app.dart';
 import '../../support/test_dependencies.dart';
 
@@ -74,7 +72,7 @@ void main() {
   late FakeNoteGateway noteGateway;
   late FakePdfSourceGateway pdfGateway;
   late FakeAssetGateway assetGateway;
-  late MemoryAssetFileCache files;
+  late MemoryFileCache files;
   late FakePdfEngine engine;
 
   setUp(() {
@@ -95,7 +93,7 @@ void main() {
     noteGateway = FakeNoteGateway();
     pdfGateway = FakePdfSourceGateway();
     assetGateway = FakeAssetGateway();
-    files = MemoryAssetFileCache();
+    files = MemoryFileCache();
     engine = FakePdfEngine();
   });
 
@@ -240,7 +238,7 @@ void main() {
       'selected, tied to the page', (tester) async {
     final assets = AssetRepository(
       assetGateway,
-      FakeAssetServerGateway(),
+      FakeFileServerGateway(),
       files,
       FakeAssetSealing(),
     );

apps/learn/halkyon_learn_flutter/test/feature/assistant/conversation_test.dart

@@ -29,14 +29,12 @@ import 'package:material_ui/material_ui.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_conversation_gateway.dart';
 import '../../support/fake_grammar_gateway.dart';
 import '../../support/fake_photo_picker.dart';
 import '../../support/fake_settings_gateway.dart';
 import '../../support/fake_studyset_gateway.dart';
-import '../../support/memory_asset_file_cache.dart';
 import '../../support/memory_device_settings_store.dart';
 import '../../support/pump_app.dart';
 import '../../support/test_dependencies.dart';
@@ -123,7 +121,7 @@ void main() {
   late MemoryDeviceSettingsStore store;
   late List<Uri> searched;
   late FakeAssetGateway assetGateway;
-  late MemoryAssetFileCache assetFiles;
+  late MemoryFileCache assetFiles;
   late FakeGrammarGateway grammar;
 
   setUp(() {
@@ -150,7 +148,7 @@ void main() {
     speech = FakeSpeechInput();
     searched = [];
     assetGateway = FakeAssetGateway();
-    assetFiles = MemoryAssetFileCache();
+    assetFiles = MemoryFileCache();
     grammar = FakeGrammarGateway(
       found: {
         '私はりんごは食べる': const [
@@ -1469,7 +1467,7 @@ void main() {
     Future<String> picturedCard() async {
       final assets = AssetRepository(
         assetGateway,
-        FakeAssetServerGateway(),
+        FakeFileServerGateway(),
         assetFiles,
         FakeAssetSealing(),
       );

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();