fix: automatic coupler crashes, horns, compressor start delay, radio and cab lever sounds

- automatic couplers don't catch above the safe collision speed (#86): with crash damage on they couple at a
  closing speed of 15 km/h at most, a faster hit locks the coupling until the vehicles part and the collision
  runs its course. On the way: CollisionVelocity() read a head-on 10 + 10 km/h as 14.1 km/h (quaternions turn
  by half the angle), it's now taken from the touching ends; CollisionDetect() took the other vehicle's speed
  the wrong way round for vehicles facing each other. Head-on hits now derail sooner, at the intended limit,
- a single horn lever can't sound both tones at once (eu07/maszyna#392),
- a skid ends once the vehicle stops on locked wheels (eu07/maszyna#479),
- electronic destination signs are blanked at the end of the timetable (eu07/maszyna#446),
- horns need the air or the voltage to sound (eu07/maszyna#290, #76): HornType=[Pneumatic]|Electric and
  Horn3Type in Cntrl.; a pneumatic horn needs 0.2 MPa in the main reservoir (or the main pipe without one),
  an electric one the low voltage; the lever stays where it was put,
- compressor start delay (#67): CompressorStartDelay [s] in Brake., counted from the compressor getting its
  power, after the engine starts say; none by default,
- the channel and volume buttons of a radio without power do nothing (eu07/maszyna#429); the power check of
  the radio is TMoverParameters::RadioPowered(),
- a lever moved smoothly sounds the positions it passes (#83).

Sonar findings in the functions changed: nesting, nested conditionals, commented-out code, std::format,
references to const. Unit tests: test::pair for two vehicles running into each other, test_couplers,
test_horn, test_wheelslip, test_compressor, test_radio, test_gauge_positions.

tests/unit/README.md

@@ -56,8 +56,10 @@ TEST_CASE("brakes: service braking stops a heavier wagon too") {
   functions on the vehicle state the class calls, and the test calls them the same way: see
   `vehicle/handover.h`, which `TController::TakeControl()` uses, and `test_handover.cpp`.
 - `vehicle` creates the vehicle the way `TDynamicObject::Init()` does and steps it the way the
-  simulation loop does, on a straight track of the given friction and gradient. Coupled vehicles
-  aren't supported yet.
+  simulation loop does, on a straight track of the given friction and gradient. `pair` puts two of
+  them on that track facing each other with the given ends and runs them together, as the
+  simulation runs the vehicles of a scene: the coupler and buffer forces, collisions and automatic
+  coupling come from the physics (`test_couplers.cpp`). Longer consists aren't supported yet.
 - Check behaviour (speeds, pressures, states the player sees) rather than internal flags, and make
   the test check its own preconditions (`REQUIRE` that the braking skidded, before checking what a
   skid leads to).