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/map.rs

@@ -60,13 +60,14 @@ pub enum Kind {
     Aerie,      // room for flying creatures (their beds)
     Steel,      // steel wall: hardly anyone gets through
     ManaSpring, // crystal spring: once claimed, more mana and a higher mana maximum
+    RuneChest,  // dwarven chest with the plans of a rune forge tower (claim it: one free build)
 }
 
 // options for the map generator (campaign missions)
 #[derive(Clone, Debug)]
 pub struct GenOpts {
     pub portals: Option<usize>,   // None: one per 64 x 64 tiles
-    pub treasures: Option<usize>, // guarded treasure caves; None: portals - 1
+    pub treasures: Option<usize>, // knight treasure caves; None: none (free play has dwarf caves instead)
     pub gate: bool,               // hero gate at the far side
     pub core_y: Option<i32>,      // row of the core (None: about the middle)
 }
@@ -88,7 +89,7 @@ pub struct River {
 }
 
 // order matters for the packed savegame codec: only append new kinds
-const KINDS: [Kind; 26] = [
+const KINDS: [Kind; 27] = [
     Kind::Rock,
     Kind::Earth,
     Kind::Gold,
@@ -115,6 +116,7 @@ const KINDS: [Kind; 26] = [
     Kind::Aerie,
     Kind::Steel,
     Kind::ManaSpring,
+    Kind::RuneChest,
 ];
 
 // treasures (veins, treasury content, imp loads): 0 gold, 1 silver, 2 ruby, 3 diamond
@@ -210,6 +212,7 @@ impl Tile {
                 | Kind::Shard
                 | Kind::Aerie
                 | Kind::ManaSpring
+                | Kind::RuneChest
         )
     }
     // river tiles (open, but nobody can walk there)
@@ -349,7 +352,7 @@ impl Map {
         let area = (w * h) as f32 / 4096.0;
         let scale = |n: f32| (n * area).round() as i32;
         let n_portals = opts.portals.unwrap_or((area.round() as usize).max(1));
-        let n_treasures = opts.treasures.unwrap_or(n_portals.saturating_sub(1));
+        let n_treasures = opts.treasures.unwrap_or(0);
 
         let mut t = vec![Tile::new(Kind::Earth); w * h];
         for (i, tile) in t.iter_mut().enumerate() {