revert kill_xp updates removal to preserve functionality of in-repo mods (#82882)

* revert kill_xp updates removal to preserve functionality of in-repo mods

* actually capture the killer pointer

* re-add savegame serialization for kill_xp

Author: Maleclypse

src/kill_tracker.cpp

@@ -104,6 +104,9 @@ static Character *get_avatar_or_follower( const character_id &id )
     return nullptr;
 }
 
+// Legacy value, maintained until kill_xp rework/removal from dependent in-repo mods
+static constexpr int npc_kill_xp = 10;
+
 void kill_tracker::notify( const cata::event &e )
 {
     switch( e.type() ) {
@@ -112,16 +115,20 @@ void kill_tracker::notify( const cata::event &e )
             if( Character *killer = get_avatar_or_follower( killer_id ) ) {
                 const mtype_id victim_type = e.get<mtype_id>( "victim_type" );
                 kills[victim_type]++;
+                // Legacy value update, maintained until kill_xp rework/removal from dependent in-repo mods
+                killer->kill_xp += e.get<int>( "exp" );
                 victim_type.obj().families.practice_kill( *killer );
             }
             break;
         }
         case event_type::character_kills_character: {
             const character_id killer_id = e.get<character_id>( "killer" );
             // player is credited for NPC kills they or their followers make
-            if( get_avatar_or_follower( killer_id ) ) {
+            if( Character *killer = get_avatar_or_follower( killer_id ) ) {
                 const std::string victim_name = e.get<cata_variant_type::string>( "victim_name" );
                 npc_kills.push_back( victim_name );
+                // Legacy value update, maintained until kill_xp rework/removal from dependent in-repo mods
+                killer->kill_xp += npc_kill_xp;
             }
             break;
         }

src/savegame_json.cpp

@@ -777,6 +777,9 @@ void Character::load( const JsonObject &data )
     data.read( "stim", stim );
     data.read( "stamina", stamina );
 
+    //legacy value, maintained until kill_xp dependency removed from in-repo mods
+    data.read( "kill_xp", kill_xp );
+
     data.read( "moncams", moncams );
 
     data.read( "magic", magic );
@@ -1411,6 +1414,9 @@ void Character::store( JsonOut &json ) const
     json.member( "type_of_scent", type_of_scent );
     json.member( "focus_pool", focus_pool );
 
+    //legacy value, maintained until kill_xp dependency removed from in-repo mods
+    json.member( "kill_xp", kill_xp );
+
     // breathing
     json.member( "underwater", underwater );
     json.member( "oxygen", oxygen );