build(train): Train on the desktop as a snap, to the Store's edge channel with each release (ADR 0030, D62)

Built as Todo's in a clone of its own, from apps/train/snap/, for train.halkyon.eu; its
listing says the desktop shows, plans and keeps the statistics, and records nothing.
The snap tests now hold every snap built in a clone of its own.

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/release.dart

@@ -32,4 +32,13 @@ const trainRelease = AppRelease(
     indexCertificateFile: 'tool/release/fdroid-index-certificate.sha256',
     repositoryUrl: 'https://halkyon.app/fdroid/repo',
   ),
+  // The desktop's Train (ADR 0030, D62), built in a clone of its own as
+  // Todo's (`SnapRelease.projectPath`), the root's snap/ being Learn's; to the
+  // Store's edge channel, its name registered on 3 Oct 2026.
+  snap: SnapRelease(
+    name: 'halkyon-train',
+    part: 'halkyon-train',
+    directory: 'apps/train/snap',
+    storeChannel: 'edge',
+  ),
 );