Steel walls: plate earth walls next to own ground with steel

- New tool Steel (key O, orders category; campaign from mission 11): 80 gold per earth wall,
  40 for a reinforced wall; riveted iron plate texture
- Heroes dig through steel 20 times slower than through earth and avoid it (path cost 300)
- Selling turns a steel wall back into a reinforced wall and refunds half the plating
- Texts EN/DE, test

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

src/gfx.rs

@@ -65,6 +65,8 @@ pub const D_DIAMOND: u32 = 123;
 pub const D_RUBBLE: u32 = 124; // floor decals: pebbles and fragments
 pub const D_CRACK: u32 = 125; // a long crack through several stones
 pub const T_AERIE: u32 = 126; // 4 variants: straw, twigs and feathers
+pub const T_STEEL: u32 = 130; // 4 variants: riveted iron plates
+pub const I_STEEL: u32 = 134;
 
 pub struct Gfx {
     pub atlas: Texture2D,
@@ -744,6 +746,31 @@ fn gen_atlas() -> Image {
             }
         });
     }
+    // steel walls: riveted plates with a little rust
+    for v in 0..4u32 {
+        cv.cell(T_STEEL + v, |x, y| {
+            let seam = x % 8 == 0 || y % 8 == 0;
+            let rivet = (x % 8 == 2 || x % 8 == 6) && (y % 8 == 2 || y % 8 == 6);
+            let rust = rnd(x / 2, y / 2, v + 240) > 0.86;
+            if seam {
+                Some(c(44, 48, 58))
+            } else if rivet {
+                Some(c(170, 176, 190))
+            } else if rust {
+                Some(shade((120, 70, 40), (rnd(x, y, v + 241) * 16.0) as i32))
+            } else {
+                Some(shade((96, 102, 116), (rnd(x, y, v + 242) * 10.0) as i32 - 5 + if y % 8 == 1 { 10 } else { 0 }))
+            }
+        });
+    }
+    cv.cell(I_STEEL, |x, y| {
+        if (2..=13).contains(&x) && (3..=12).contains(&y) {
+            let rivet = (x == 4 || x == 11) && (y == 5 || y == 10);
+            Some(if rivet { c(220, 224, 235) } else if x == 2 || y == 3 || x == 13 || y == 12 { c(60, 64, 76) } else { c(130, 136, 150) })
+        } else {
+            None
+        }
+    });
     // floor decals
     cv.cell(D_RUBBLE, |x, y| {
         for k in 0..9 {
@@ -1229,6 +1256,7 @@ impl Gfx {
             Kind::LavaBridge => T_STONEBRIDGE + (v & 1),
             Kind::Shard => T_HEROFLOOR + v,
             Kind::Aerie => T_AERIE + v,
+            Kind::Steel => T_STEEL + v,
         }
     }