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/save.rs

@@ -98,7 +98,8 @@ mod tests {
         let mut g = Game::new(7, 256, 256);
         let gen = t0.elapsed();
         assert_eq!(g.map.portals.len(), 16);
-        assert_eq!(g.map.treasures.len(), 15);
+        assert_eq!(g.map.treasures.len(), 0); // knight caves are replaced by dwarf caves
+        assert!((1..=4).contains(&g.dwarf_caves.len()));
         let (cx, cy) = g.map.core;
         g.mark_rect(cx - 5, cy - 5, cx + 8, cy + 8, true);
         let t1 = std::time::Instant::now();