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/gfx.rs

@@ -69,6 +69,7 @@ pub const T_STEEL: u32 = 130; // 4 variants: riveted iron plates
 pub const I_STEEL: u32 = 134;
 pub const T_RIVAL1: u32 = 135; // 4 variants: floor of the green master
 pub const T_RIVAL2: u32 = 139; // 4 variants: floor of the blue master
+pub const I_RUNE: u32 = 143; // rune forge tower
 
 pub struct Gfx {
     pub atlas: Texture2D,
@@ -570,6 +571,23 @@ fn gen_atlas() -> Image {
             None
         }
     });
+    cv.cell(I_RUNE, |x, y| {
+        // a stone obelisk with a glowing rune, bronze bands
+        let body = (5..=10).contains(&x) && y >= 3 && y <= 14 && !(y == 3 && (x == 5 || x == 10));
+        if !body {
+            return None;
+        }
+        let rune = (x == 7 || x == 8) && (6..=10).contains(&y) || (y == 8 && (6..=9).contains(&x));
+        Some(if rune {
+            c(90, 210, 255)
+        } else if y == 4 || y == 12 {
+            c(190, 120, 60)
+        } else if (x + y) % 5 == 0 {
+            c(80, 80, 92)
+        } else {
+            c(110, 110, 124)
+        })
+    });
     cv.cell(I_SUMMON, |x, y| {
         let d = (((x - 8) * (x - 8) + (y - 8) * (y - 8)) as f32).sqrt();
         let a = ((y - 8) as f32).atan2((x - 8) as f32);
@@ -1383,6 +1401,7 @@ impl Gfx {
             Kind::Aerie => T_AERIE + v,
             Kind::Steel => T_STEEL + v,
             Kind::ManaSpring => T_PORTALFLOOR + v,
+            Kind::RuneChest => T_FLOOR + v,
         }
     }