Editor: fold the plan library into the tree as modules, drop its own name

The track layout library came in as a subproject of its own -- its own
CMakeLists, its own install and export rules, its own include tree -- which only
made the build harder to follow now that everything else is a module. It is part
of the editor, so it lives where the editor lives.

- editor/plan_*.cppm / plan_*.cpp: the fifteen interface units and twelve
  implementation units, flat in editor/ beside editorTerrain and orthophoto, with
  the plan_ prefix keeping them clear of what was already there (editor/turnouts
  is the preset catalogue, so the geometry is plan_turnout_geometry).
- modules are named eu07.editor.plan_*, like every other module in the tree.
- namespaces move under the editor's own: editor::plan, editor::plan::geometry,
  ::io, ::render. The library's old name is gone from the sources and the docs.
- every moved file carries the MPL header the rest of the tree carries; it had
  none. version.hpp went with the subproject: macros only, and unused.
- the umbrella header is dropped -- nothing imported it.
- the unit tests survive the fold: editor/tests, behind EU07_EDITOR_PLAN_TESTS
  (off by default), compiles the interfaces a second time into a small library of
  its own, because a test can only import modules from a target it links and
  linking the game would drag the whole simulator in. The code under test needs
  nothing but a compiler. 7/7 pass.
- a switch may now be written with two dashes as well: --editor beside -editor.

Builds with clang 22 (-DWITH_BETTER_RENDERER=OFF); -editor and --editor both come
up in the editor on empty ground.

editor/plan_join.cppm

@@ -0,0 +1,75 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+module;
+#include <string>
+#include <vector>
+
+export module eu07.editor.plan_join;
+export import eu07.editor.plan_segment;
+export import eu07.editor.plan_model;
+export import eu07.editor.plan_solution;
+
+export {
+
+// TYMCZASOWA ZASLEPKA. Modul scalania koncow (plan_join / join_ends) nie zostal nigdy
+// zacommitowany: commit 4ddfeb02 dopisal `src/editor/join.cpp` do CMakeLists i `#include` w
+// planpanel.cpp, ale samych plikow nie ma ani w drzewie, ani w historii, ani w stashach. Bez
+// tego naglowka i join.cpp branch sie nie kompiluje wcale. Ta zaslepka odtwarza dokladnie to
+// API, ktorego uzywa panel planu, i mowi "niedostepne" - zeby reszta edytora dala sie uruchomic.
+// Do zastapienia prawdziwym rozwiazaniem geometrii (luk-prosta-luk) i chirurgia dokumentu.
+
+
+
+namespace editor::plan {
+
+/// Co stanelo miedzy dwoma koncami.
+enum class JoinKind {
+    None,
+    Straight,
+    Arc,
+    ArcLineArc,
+    ArcArc,
+};
+
+[[nodiscard]] const char* join_kind_name(JoinKind kind);
+
+/// Czym wolno gietc to, co lezy miedzy koncami.
+struct JoinSettings {
+    double radius{300.0};
+};
+
+/// Propozycja, przed tknieciem dokumentu.
+struct JoinPlan {
+    bool ok{false};
+    std::string why{};
+    JoinKind kind{JoinKind::None};
+    std::vector<Element> elements{};
+    double tightest{0.0};
+};
+
+[[nodiscard]] JoinPlan plan_join(const geometry::Pose& leaving,
+                                 const geometry::Pose& arriving,
+                                 const JoinSettings& settings);
+
+/// Co scalenie kosztowalo.
+struct JoinReport {
+    bool ok{false};
+    std::string why{};
+    JoinPlan join{};
+    int fused{0};
+    int turnouts{0};
+    bool reversed{false};
+};
+
+[[nodiscard]] JoinReport join_ends(Document& document, const Solution& solution, TrackId a,
+                                   int a_end, TrackId b, int b_end, const JoinSettings& settings);
+
+}  // namespace editor::plan
+
+}  // export