fix(ai): release horn when human driver takes over from AI (#210)

* fix(ai): release what the AI was holding when the player takes over

* fix(ai): keep the light selector lookup within its table, Sonar findings of the handover

- the light selector size comes from the vehicle definition (LightsList Size)
  and isn't limited, while the table holds 18 positions; a bigger selector
  read past the table, and so did a table index other than the front or
  rear one. Only the positions the table holds are looked at now, and the
  selector stays where it is for a bad table index (cpp:S2259),
- the lambdas of the handover capture what they use explicitly (cpp:S3608),
- the chained assignments resetting the headlight state on the AI taking
  over are split (cpp:S1121).

tests/unit/README.md

@@ -52,6 +52,9 @@ TEST_CASE("brakes: service braking stops a heavier wagon too") {
   and a test on a half-defined vehicle tests the definition rather than the simulation.
 - The random numbers of the simulation are reseeded before every test case, so a test gives the
   same result alone or among the others.
+- Logic which lives in classes too big to set up in a test (the AI driver, the cab) goes into free
+  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.