refactor(track): the sleeper layout split by concern instead of one TrackSleepers.cpp

- SleeperRules: the rules beams are laid by and the faults counted where the layout breaks them
- SleeperGeometry: plan helpers, beam boxes with their overlap and support tests and a grid of cells, shared by laying, filling
  gaps, keeping beams apart and checking instead of three copies of each
- SleeperPaths: sampled paths, how near they run and whether one carries another; the tracks near a track and the stretches of
  a path where a test holds, shared by paths_around/unsettle_around and shared_zones/zones_of
- SleeperFeatures: frogs and crossings the timbers turn to; SleeperLines: lines of beams, the rails they carry, where they part
- SleeperLayout: clusters and their layout, given the epoch instead of reading it; SleeperModels: the models of the beams
- SleeperWorker: the layouts kept per epoch and worked out in the background, behind a small interface
- TrackSleepers: the facade, laying the instances, the shared zones and the check
- sleepercheck reports a digest of all beams; geomhash skips empty chunks instead of crashing

The layout is unchanged: geomhash, sleepercheck with the digest, wings, space, guards, crossings and switches give the same
output on baltyk, zz_rozjazdy_id4, zz_rf_slips, zz_rf_wide, zz_slips_s42, zz_demo_przechylka and zz_connections.

world/SleeperWorker.h

@@ -0,0 +1,31 @@
+/*
+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/.
+*/
+
+#pragma once
+
+#include <optional>
+#include <vector>
+
+#include "world/TrackSleepers.h"
+
+// the layouts of the clusters of tracks kept till a track changes, and worked out in a thread of their own while the simulation runs
+namespace track_sleepers {
+
+// the layouts worked out so far hold no more, as a track changed
+void outdate_layouts();
+// the beams of the track from the layout of its cluster worked out already for the tracks as they are, none where there's none yet
+std::optional<std::vector<beam>> laid_out( TTrack const &Track );
+// whether the layouts are worked out in the background
+bool in_background();
+// the layout of the cluster of the track asked of the thread working them out
+void ask_layout( TTrack &Track );
+// nothing left to work out in the background nor being worked out, the waiting ones dropped
+void settle_background();
+
+} // namespace track_sleepers