Give different Heretic episodes different default themes.  Each theme
appears twice.

diff -ur Oblige-7.70-bright/games/heretic/levels.lua Oblige-7.70-ObHack/games/heretic/levels.lua
--- Oblige-7.70-bright/games/heretic/levels.lua	2026-07-18 23:12:14.584466500 -0600
+++ Oblige-7.70-ObHack/games/heretic/levels.lua	2026-08-10 20:57:34.360380400 -0600
@@ -28,7 +28,7 @@
   {
     ep_index = 1
 
-    theme = "castle"
+    theme = "town"
     boss = "Ironlich"
     sky_light = 0.95
   }
@@ -37,7 +37,7 @@
   {
     ep_index = 2
 
-    theme = "castle"
+    theme = "egypt"
     boss = "Maulotaur"
     sky_light = 0.95
   }
@@ -55,7 +55,7 @@
   {
     ep_index = 4
 
-    theme = "castle"
+    theme = "egypt"
     boss = "Ironlich"
     sky_light = 0.95
   }
@@ -83,7 +83,7 @@
   {
     ep_index = 6
 
-    theme = "castle"
+    theme = "town"
     boss = "Ironlich"
     sky_light = 0.70
   }