Gate 3+ engines behind DEBUG_HS (#45914)

It's not intended that a survivor can install more than 2 engines in a
vehicle, yet it is still useful to be able to install more than 2
engines for vehicle creation.

Additionally, it was often asked how to get to mechanics 12, an
effectively impossible task, but still show for the install
requirements for 2+ engines.

As such, require debug hammerspace to install more than 2 engines or
display the skills need to install more engines.
The skills don't mean anything after 2 engines, but it's easier to leave
them there, and there is no harm in doing so.

src/veh_interact.cpp

@@ -795,14 +795,21 @@ bool veh_interact::update_part_requirements()
 
     nmsg += _( "<color_white>Additional requirements:</color>\n" );
 
+    bool allow_more_eng = engines < 2 || player_character.has_trait( trait_DEBUG_HS );
+
     if( dif_eng > 0 ) {
-        if( player_character.get_skill_level( skill_mechanics ) < dif_eng ) {
+        if( !allow_more_eng || player_character.get_skill_level( skill_mechanics ) < dif_eng ) {
             ok = false;
         }
-        //~ %1$s represents the internal color name which shouldn't be translated, %2$s is skill name, and %3$i is skill level
-        nmsg += string_format( _( "> %1$s%2$s %3$i</color> for extra engines." ),
-                               status_color( player_character.get_skill_level( skill_mechanics ) >= dif_eng ),
-                               skill_mechanics.obj().name(), dif_eng ) + "\n";
+        if( allow_more_eng ) {
+            //~ %1$s represents the internal color name which shouldn't be translated, %2$s is skill name, and %3$i is skill level
+            nmsg += string_format( _( "> %1$s%2$s %3$i</color> for extra engines." ),
+                                   status_color( player_character.get_skill_level( skill_mechanics ) >= dif_eng ),
+                                   skill_mechanics.obj().name(), dif_eng ) + "\n";
+        } else {
+            nmsg += _( "> <color_red>You cannot install any more engines on this vehicle.</color>" ) +
+                    std::string( "\n" );
+        }
     }
 
     if( dif_steering > 0 ) {
@@ -2059,8 +2066,22 @@ int veh_interact::part_at( const point &d )
  */
 bool veh_interact::can_potentially_install( const vpart_info &vpart )
 {
-    return get_player_character().has_trait( trait_DEBUG_HS ) ||
-           vpart.install_requirements().can_make_with_inventory( crafting_inv, is_crafting_component );
+    bool engine_reqs_met = true;
+    bool can_make = vpart.install_requirements().can_make_with_inventory( crafting_inv,
+                    is_crafting_component );
+    bool hammerspace = get_player_character().has_trait( trait_DEBUG_HS );
+
+    int engines = 0;
+    if( vpart.has_flag( VPFLAG_ENGINE ) && vpart.has_flag( "E_HIGHER_SKILL" ) ) {
+        for( const vpart_reference &vp : veh->get_avail_parts( "ENGINE" ) ) {
+            if( vp.has_feature( "E_HIGHER_SKILL" ) ) {
+                engines++;
+            }
+        }
+        engine_reqs_met = engines < 2;
+    }
+
+    return hammerspace || ( can_make && engine_reqs_met );
 }
 
 /**