feat(editor): a track deleted cuts the design kept for its line down to the tracks left along it, in the same step of the undo history: a design keeps the stretch of its line it stands for (//$h kept <from> <to>), a track deleted in the middle parts it in two, one at an end shortens it, a design with no track left is forgotten. The geometry stays as designed, only its reach changes. A design cut down isn't taken whole by the route plan nor merged with a new one nor carried along by a switch slid, and gives no designed curve to the tracks left of a curve it cut, which are recognized as they lie: a track laid by hand into the gap is never pulled back to the old shape. The status tells of it. The //$h click lines are the editor's own now: read back with their design, and written once on save instead of staying behind and piling up. Selftests on a copy of wgr.scn: a track of a compound curve deleted in the middle, undone and redone, one at an end, all of a piece, saved and read back

application/editormode.h

@@ -1780,6 +1780,7 @@ class editor_mode : public application_mode, private editor_track::observer, pri
 	void render_straight_ui();
 	track_tab m_track_tab{track_tab::path};
 	void delete_selected_track();
+	std::string designs_trimmed(std::vector<TTrack *> const &Removed);
 	void start_straight_gesture(int const Tool);
 	void finish_straight_gesture();
 	void toggle_straight_set(TTrack &Track);