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/pubspec.yaml

@@ -17,6 +17,9 @@ dependencies:
   # What a route's map kept offline was made for (ADR 0030, D45).
   crypto: ^3.0.7
   equatable: ^3.0.0
+  # A GPX file brought in (ADR 0030, D45), the platform's own dialog, as
+  # Learn and halkyon_app open and save files.
+  file_selector: ^1.1.0
   flutter:
     sdk: flutter
   flutter_bloc: ^9.1.1
@@ -65,6 +68,8 @@ dependencies:
   # Reads a style kept offline (D45), which vector_map_tiles reads only from
   # the network; the version vector_map_tiles takes.
   vector_tile_renderer: ^6.1.0
+  # Routes as GPX, written and read (D45).
+  xml: ^7.0.1
 
 dev_dependencies:
   flutter_driver: