Keep one primary per consist and let a secondary AI claim it.

A driver without movePrimary no longer just idles. UpdateSecondary() cuts
BrakeValveActive so a secondary AI stops fighting the primary over the brake
pipe, and - as long as no driver in the consist has iEngineActive - keeps
scanning ahead of CabOccupied, so it can take movePrimary and start the engine
once a signal permits.

scan_route() orients by CabOccupied when DirActive is 0, and
ClaimConsistPrimary() clears movePrimary on every other driver in the consist,
then calls TableClear() and CheckVehicles(), so sSpeedTable is rebuilt for the
new front and ctOwner follows it.

- walk the consist with FirstFind()/Neighbour() instead of Next()/Prev(), which
  stopped at the starting vehicle and mishandled reversed members;
- ReassignConsistPrimary() picks a single driver after any coupling change,
  both from TDynamicObject::couple() and from the AI path;
- IsOccupiedByAnotherConsist() decides membership with BelongsToConsist() and
  takes a signed distance, so a consist left behind no longer cuts the scan
  short. DistanceToAnotherConsist() hands back the gap it already computes, so
  OtherConsistAtSignal() counts a foreign vehicle only while it stands closer
  than the signal itself, instead of comparing track starts;
- TSpeedPos::Set() marks an entry with spNotOurShuntSignal when a
  spShuntSemaphor was already permitting as the entry was added. A shunting
  signal does not address its permission to a particular consist, so a dormant
  driver does not wake up on a permission it did not see light up; the flag
  drops once the signal shows stop, so its own turn is never blocked. spSemaphor
  is excluded: a semaphore sets a route for us and permits us even when found
  already lit. The check reads SemPermitIndex - the entry that produced the
  permitting command - because SemNextIndex can point at an earlier signal.
  The flag is also stamped whenever the permission turns out to belong to a
  consist standing in front of us - both where TableUpdateEvent() already knows
  the signal is for the vehicle ahead, and in check_route_ahead() - so the
  addressee pulling away does not hand the still-lit signal over to whoever
  waited behind it;
- the scantable debug panel lists the flags by name (spNotOurShuntSignal shows
  as not-ours) and folds the nearest vehicle ahead into the table by its own
  distance.

vehicle/DynObj.cpp

@@ -1616,6 +1616,8 @@ int TDynamicObject::Dettach(int dir)
             d = d->Next(); // i w drugą stronę
         }
     }
+    // the neighbour before uncoupling - afterwards Neighbours no longer points at it
+    auto *const severedneighbour { MoverParameters->Neighbours[ dir ].vehicle };
     if( MoverParameters->Couplers[ dir ].CouplingFlag ) {
         // odczepianie, o ile coś podłączone
         MoverParameters->Dettach( dir );
@@ -1628,10 +1630,54 @@ int TDynamicObject::Dettach(int dir)
         };
 */
     }
+    // after uncoupling every part of the consist must have at most one primary
+    ReassignConsistPrimary();
+    if( severedneighbour != nullptr ) {
+        severedneighbour->ReassignConsistPrimary();
+    }
     // sprzęg po rozłączaniu (czego się nie da odpiąć
     return MoverParameters->Couplers[dir].CouplingFlag;
 }
 
+void TDynamicObject::ReassignConsistPrimary()
+{ // picks the primary; ClaimConsistPrimary does the actual primary / valve / table change
+    std::vector<TController *> drivers;
+    TController *existingprimary = nullptr;
+    int primarycount = 0;
+    // walk from the front; Next()/Prev() are not enough, reversed members need the coupler side flipped
+    int side { end::front };
+    auto *front { FirstFind( side ) };
+    if( front == nullptr ) { front = this; }
+    auto dir { 1 - side };
+    auto *farend { front };
+    for( auto *d = front; d != nullptr; d = d->Neighbour( dir ) )
+    {
+        if( d->Mechanik != nullptr )
+        {
+            drivers.emplace_back( d->Mechanik );
+            if( d->Mechanik->primary() ) {
+                ++primarycount;
+                existingprimary = d->Mechanik;
+            }
+        }
+        farend = d;
+    }
+    if( drivers.empty() ) { return; }
+    // a human-driven cab takes priority, so the AI does not take the consist from them
+    TController *humandriver = nullptr;
+    for( auto *drv : drivers ) {
+        if( false == drv->AIControllFlag ) { humandriver = drv; break; }
+    }
+    // with a single primary it stays; otherwise: the human, the end vehicles, the first on the list
+    TController *newprimary =
+        ( primarycount == 1 )           ? existingprimary :
+        ( humandriver != nullptr )      ? humandriver :
+        ( front->Mechanik != nullptr )  ? front->Mechanik :
+        ( farend->Mechanik != nullptr ) ? farend->Mechanik :
+                                          drivers.front();
+    newprimary->ClaimConsistPrimary();
+}
+
 void
 TDynamicObject::couple( int const Side ) {
 
@@ -1653,6 +1699,7 @@ TDynamicObject::couple( int const Side ) {
                     Side, neighbour.vehicle_end,
                     othervehicleparams,
                     coupling::coupler ) ) {
+                ReassignConsistPrimary(); // coupled consists are one consist, with one primary
                 // one coupling type per key press
                 return;
             }
@@ -3118,6 +3165,7 @@ bool TDynamicObject::Update(double dt, double dt1)
         MoverParameters->InsideConsist = false;
     }
     if( TestFlag( MoverParameters->AIFlag, sound::attachcoupler ) ) {
+        ReassignConsistPrimary(); // coupled consists are one consist, with one primary
         auto *driver{ ctOwner ? ctOwner : Mechanik };
         if( driver != nullptr ) {
             driver->CheckVehicles( Connect );