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>

tool/app_icons.py

@@ -97,6 +97,7 @@ APPS = {
         "halkyon_train_icon.png",
         "dumbbell",
         (COBALT, VIOLET, STRAWBERRY, CARROT),
+        snap="apps/train/snap/gui/halkyon-train.png",
     ),
 }