Wilder maps: rivers of water and lava across free maps (neutral bridges), natural caverns until about a quarter of every map is open

- caverns in free play and the campaign (mission pieces, fortresses, islands, sealed caves and dwarf caves stay untouched)
- caverns sometimes hold a pool of water or lava, veins in their walls, rarely a small treasure or a creature in chains to free
- rivers keep away from the start; test: open share, liquids and a hero path to the core
- standalone file in play/ rebuilt

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

src/campaign.rs

@@ -504,6 +504,7 @@ pub fn start_mission(i: usize, carry: Option<&Carry>) -> Game {
         g.add_rival(side, x, y, diff);
     }
     g.place_dwarf_caves(d.seed);
+    g.add_wilds(d.seed, false);
     g.init_research(Some(i));
     g.init_fog();
     let heart = d.gate || d.forts.iter().any(|f| f.heart);