Make infestation scarab-creation into a fineff (#10848)

To fix newly-created scarabs being turned hostile when killing their host in
an explosion.

crawl-ref/source/fineff.cc

@@ -529,6 +529,25 @@ void bennu_revive_fineff::fire()
         newmons->props["bennu_revives"].get_byte() = revives + 1;
 }
 
+void infestation_death_fineff::fire()
+{
+    if (monster *scarab = create_monster(mgen_data(MONS_DEATH_SCARAB,
+                                                   BEH_FRIENDLY, posn,
+                                                   MHITYOU, MG_AUTOFOE)
+                                         .set_summoned(&you, 0,
+                                                       SPELL_INFESTATION),
+                                         false))
+    {
+        scarab->add_ench(mon_enchant(ENCH_FAKE_ABJURATION, 6));
+
+        if (you.see_cell(posn) || you.can_see(*scarab))
+        {
+            mprf("%s bursts from %s!", scarab->name(DESC_A, true).c_str(),
+                                       name.c_str());
+        }
+    }
+}
+
 // Effects that occur after all other effects, even if the monster is dead.
 // For example, explosions that would hit other creatures, but we want
 // to deal with only one creature at a time, so that's handled last.

crawl-ref/source/fineff.h

@@ -319,6 +319,24 @@ class bennu_revive_fineff : public final_effect
     unsigned short foe;
 };
 
+class infestation_death_fineff : public final_effect
+{
+public:
+    bool mergeable(const final_effect &a) const override { return false; }
+    void fire() override;
+
+    static void schedule(coord_def pos, const string &name)
+    {
+        final_effect::schedule(new infestation_death_fineff(pos, name));
+    }
+protected:
+    infestation_death_fineff(coord_def pos, const string &_name)
+        : final_effect(0, 0, pos), name(_name)
+    {
+    }
+    string name;
+};
+
 void fire_final_effects();
 
 #endif

crawl-ref/source/mon-death.cc

@@ -1424,22 +1424,8 @@ static bool _explode_monster(monster* mons, killer_type killer,
 
 static void _infestation_create_scarab(monster* mons)
 {
-    if (monster *scarab = create_monster(mgen_data(MONS_DEATH_SCARAB,
-                                                   BEH_FRIENDLY, mons->pos(),
-                                                   MHITYOU, MG_AUTOFOE)
-                                         .set_summoned(&you, 0,
-                                                       SPELL_INFESTATION),
-                                         false))
-    {
-        scarab->add_ench(mon_enchant(ENCH_FAKE_ABJURATION, 6));
-
-        if (you.see_cell(mons->pos()) || you.can_see(*scarab))
-        {
-            mprf("%s bursts from %s!", scarab->name(DESC_A, true).c_str(),
-                                       mons->name(DESC_THE).c_str());
-        }
-        mons->flags |= MF_EXPLODE_KILL;
-    }
+    mons->flags |= MF_EXPLODE_KILL;
+    infestation_death_fineff::schedule(mons->pos(), mons->name(DESC_THE));
 }
 
 static void _monster_die_cloud(const monster* mons, bool corpse, bool silent,