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_chain.cppm

@@ -0,0 +1,43 @@
+/*
+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 <vector>
+
+export module eu07.editor.plan_chain;
+export import eu07.editor.plan_segment;
+export import eu07.editor.plan_model;
+export import eu07.editor.plan_solution;
+
+export {
+
+namespace editor::plan {
+
+/// What laying one track produced, and everything that went wrong doing it.
+struct TrackOutcome {
+    SolvedTrack track;
+    std::vector<Diagnostic> diagnostics;
+};
+
+/// Lays @p track from @p start.
+///
+/// Every element states its own length, so laying is a walk: each element starts
+/// on the pose the one before it ended on, and nothing is solved for. A clothoid
+/// takes its near-end curvature from where the chain has got to, which is the
+/// only way one element depends on another.
+///
+/// Never throws. An element that cannot be laid — a non-positive length, an arc
+/// without a radius or a hand, a clothoid that changes nothing — is reported and
+/// ends the chain there; everything before it is still returned and still
+/// drawable.
+[[nodiscard]] TrackOutcome lay_track(const Document& document, const Track& track,
+                                     const geometry::Pose& start);
+
+}  // namespace editor::plan
+
+}  // export