Dwarf caves in all modes: neutral dwarves guard a hoard and sometimes a rune chest; the rune forge tower; imps repair towers before claiming

- 1-4 cave slots by map size, 50% each (at least one); knight treasure caves only where a mission places them
- dwarves fight intruders near the cave, never chase far, heal at home, grow with distance and the player's army, never count for victory
- rune chest: one free rune forge tower (chain bolt to three more enemies), upgradeable for gold, sold once it comes back (next mission / free play at once), unbuilt ones carry over
- new tool (key N) shown in the tower menu while a rune tower can be built or upgraded

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/rival.rs

@@ -85,6 +85,7 @@ impl Game {
         g.msg("msg.welcome1", vec![]);
         g.msg("msg.welcome2", vec![]);
         g.msg(if rivals == 0 { "msg.welcome_humans" } else if humans { "msg.welcome_both" } else { "msg.welcome_rivals" }, vec![rivals.to_string()]);
+        g.place_dwarf_caves(seed);
         g
     }
 
@@ -267,7 +268,7 @@ impl Game {
         }
     }
 
-    fn eliminate_rival(&mut self, k: usize) {
+    pub(crate) fn eliminate_rival(&mut self, k: usize) {
         let side = self.rivals[k].side;
         let owner = Owner::of_side(side);
         self.rivals[k].alive = false;