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/settings.rs

@@ -7,7 +7,7 @@ use std::collections::BTreeMap;
 // Rebindable keyboard actions. Esc (menu) and the arrow keys (camera) are fixed.
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub enum Action {
-    Tool(usize), // 0..19, same order as ui::TOOLS
+    Tool(usize), // 0..20, same order as ui::TOOLS
     CamForward,
     CamBack,
     CamLeft,
@@ -25,7 +25,7 @@ pub enum Action {
 }
 
 pub fn all_actions() -> Vec<Action> {
-    let mut v: Vec<Action> = (0..19).map(Action::Tool).collect();
+    let mut v: Vec<Action> = (0..20).map(Action::Tool).collect();
     v.extend_from_slice(&[
         Action::CamForward,
         Action::CamBack,
@@ -70,7 +70,7 @@ impl Action {
     fn default_key(self) -> KeyCode {
         use KeyCode::*;
         match self {
-            Action::Tool(i) => [Key1, Key2, Key3, Key4, Key5, Key6, Key7, Key8, Key9, Key0, X, V, K, J, B, C, T, U, Y][i.min(18)],
+            Action::Tool(i) => [Key1, Key2, Key3, Key4, Key5, Key6, Key7, Key8, Key9, Key0, X, V, K, J, B, C, T, U, Y, O][i.min(19)],
             Action::CamForward => W,
             Action::CamBack => S,
             Action::CamLeft => A,