Rivers and bridges: water and lava tiles, bridge tool, burning in lava

- New tile kinds (appended for the savegame codec): water, lava, wooden bridge, stone bridge, core shard
- Sunken river beds with banks, bridge decks with rails, lava glow and lights
- Bridge tool (key U, rooms category): bridges grow from own ground, 30 gold/tile over water, 75 over lava
- Selling a bridge drops the units on it into the river (water: swim to the bank, lava: burn)
- Cannon blasts and punches knock units back; into lava = burnt (heroes still pay their bounty)
- Heroes cannot path through rivers; texts EN/DE; tests for bridges and lava

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..17, same order as ui::TOOLS
+    Tool(usize), // 0..18, 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..17).map(Action::Tool).collect();
+    let mut v: Vec<Action> = (0..18).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][i.min(16)],
+            Action::Tool(i) => [Key1, Key2, Key3, Key4, Key5, Key6, Key7, Key8, Key9, Key0, X, V, K, J, B, C, T, U][i.min(17)],
             Action::CamForward => W,
             Action::CamBack => S,
             Action::CamLeft => A,