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/fortress.rs

@@ -134,7 +134,7 @@ impl Game {
             let id = self.next_id;
             self.next_id += 1;
             let s = tower_stats(kind);
-            let mut tw = Tower { id, kind, tile: i, level: 1, hp: s.hp, cd: 1.0, aim: 0.0, flash: 0.0, hero: true };
+            let mut tw = Tower { id, kind, tile: i, level: 1, hp: s.hp, cd: 1.0, aim: 0.0, flash: 0.0, hero: true, rune_final: false };
             while tw.level < f.tower_level.min(3) {
                 let old = tw.max_hp();
                 tw.level += 1;