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/main.rs

@@ -63,7 +63,7 @@ struct Input {
 }
 
 fn area_tool(t: Tool) -> bool {
-    matches!(t, Tool::Dig | Tool::Room(_) | Tool::Sell)
+    matches!(t, Tool::Dig | Tool::Room(_) | Tool::Sell | Tool::Bridge)
 }
 
 // Use the active tool on the world at (mx, my) (CSS pixels): a click, or a tap on touch screens
@@ -76,7 +76,7 @@ fn world_action(app: &mut App, ui: &mut Ui, r3d: &R3d, inp: &mut Input, mx: f32,
     let ground = app.cam.ground_point(mx, my);
     let (wx, wy) = (ground.x, ground.y);
     match (ui.tool, tile) {
-        (Tool::Dig | Tool::Room(_) | Tool::Sell, Some(t)) => inp.ldown = Some(t),
+        (Tool::Dig | Tool::Room(_) | Tool::Sell | Tool::Bridge, Some(t)) => inp.ldown = Some(t),
         (Tool::Hand, _) => {
             let j = r3d.pick_ent(&app.game, &app.cam, mx, my, false);
             if slap {
@@ -108,6 +108,7 @@ fn finish_rect(app: &mut App, tool: Tool, start: (i32, i32), mx: f32, my: f32) {
         }
         Tool::Room(r) => app.game.build_rect(r, sx, sy, ex, ey),
         Tool::Sell => app.game.sell_rect(sx, sy, ex, ey),
+        Tool::Bridge => app.game.build_bridges(sx, sy, ex, ey),
         _ => {}
     }
 }
@@ -812,6 +813,7 @@ fn draw_frame(app: &mut App, gfx: &Gfx, r3d: &mut R3d, ui: &mut Ui, menu: &mut M
         Tool::Lightning => (Color::new(1.0, 1.0, 0.6, 1.0), Some((ground.x, ground.y, 1.6))),
         Tool::Heal => (Color::new(0.4, 1.0, 0.5, 1.0), Some((ground.x, ground.y, 2.55))),
         Tool::Summon => (Color::new(0.8, 0.5, 1.0, 1.0), None),
+        Tool::Bridge => (Color::new(0.9, 0.7, 0.4, 1.0), None),
         Tool::Tower(_) => (Color::new(1.0, 0.6, 0.25, 1.0), None),
     };
     let ov = Overlay {