Editor checkpoint: turnout domain service, web->editor rename, Id-1 templates

Checkpoint of the editor branch so work isn't carried loose across a branch
switch. Builds.

- maj0sted turnout geometry moved into the domain: domain::lay_turnout lays the
  diverging path (pre-blade lead, blade at beta, arc of alfa-beta, frog rail) and
  solve_junction is a thin adapter onto it.
- the maj0sted web/ tree renamed to editor/ (namespace and paths) - there is no
  WASM here, it's plain native C++ the editor calls.
- click places the switch at its start (początek rozjazdu); the branch's leaving
  straight is locked tangent to the frog; switches can be slid along the through
  track and flipped L/P.
- Polish turnout templates carry catalogue length (Id-1-consistent lengths), plus
  blade-angle and pre-blade fields (left 0 until the catalogue figures are in).
- TURNOUT_SPEC.md: the Bezier-based runtime/model contract.
- also folds in other pending branch changes (scenery, tests) to leave a clean tree.

scene/scenenodegroups.h

@@ -34,6 +34,10 @@ public:
     // indicates creation of current group ended. returns: handle to the parent group or null_handle if group stack is empty
     group_handle
         close();
+    // drops every group without touching node pointers — call when the region that owned
+    // those nodes is about to be (or has just been) destroyed, before a scenery reload
+    void
+        clear();
 	// update minimap objects
 	void
 	    update_map();