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.cpp

@@ -0,0 +1,64 @@
+/*
+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>
+
+module eu07.editor.plan_join;
+
+// TYMCZASOWA ZASLEPKA - patrz komentarz w join.hpp.
+
+
+namespace editor::plan {
+
+namespace {
+constexpr const char* kMissing = "scalanie koncow niedostepne: brak modulu join";
+}  // namespace
+
+const char* join_kind_name(JoinKind kind) {
+    switch (kind) {
+        case JoinKind::Straight:
+            return "prosta";
+        case JoinKind::Arc:
+            return "luk";
+        case JoinKind::ArcLineArc:
+            return "luk-prosta-luk";
+        case JoinKind::ArcArc:
+            return "luk-luk";
+        case JoinKind::None:
+            break;
+    }
+    return "nic";
+}
+
+JoinPlan plan_join(const geometry::Pose& leaving, const geometry::Pose& arriving,
+                   const JoinSettings& settings) {
+    (void)leaving;
+    (void)arriving;
+    (void)settings;
+    JoinPlan plan;
+    plan.why = kMissing;
+    return plan;
+}
+
+JoinReport join_ends(Document& document, const Solution& solution, TrackId a, int a_end, TrackId b,
+                     int b_end, const JoinSettings& settings) {
+    (void)document;
+    (void)solution;
+    (void)a;
+    (void)a_end;
+    (void)b;
+    (void)b_end;
+    (void)settings;
+    JoinReport report;
+    report.why = kMissing;
+    return report;
+}
+
+}  // namespace editor::plan