feat(train): routes out and in as GPX (ADR 0030, D45)

A route's page shares it as GPX 1.1 (Android's share sheet, else saved
where the person chooses): its way a track with elevations, its
waypoints and turns as named points. A GPX file comes in from the Routes
tab or, on Android, opened with Train or shared to it; it is planned
along the paths through its points or kept as drawn, then named and
saved as a new route.

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/train/halkyon_train_flutter/test/support/fake_route_files.dart

@@ -0,0 +1,31 @@
+import 'dart:async';
+import 'dart:typed_data';
+
+import 'package:halkyon_train_flutter/src/feature/route/data/route_files.dart';
+
+/// [RouteFiles] in memory: the file [picked] next, files handed over with
+/// [open], and the files shared kept in [shared].
+class FakeRouteFiles({@override final bool shares = true})
+    implements RouteFiles {
+  final _opened = StreamController<OpenedFile>.broadcast();
+
+  /// What [pick] gives next; null: the person cancels.
+  OpenedFile? picked;
+
+  final shared = <OpenedFile>[];
+
+  /// A file opened with Train or shared to it.
+  void open(OpenedFile file) => _opened.add(file);
+
+  @override
+  Stream<OpenedFile> get opened => _opened.stream;
+
+  @override
+  Future<OpenedFile?> pick() async => picked;
+
+  @override
+  Future<bool> share({required String name, required Uint8List bytes}) async {
+    shared.add((name: name, bytes: bytes));
+    return true;
+  }
+}