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/rival.rs

@@ -85,6 +85,7 @@ impl Game {
         g.msg("msg.welcome1", vec![]);
         g.msg("msg.welcome2", vec![]);
         g.msg(if rivals == 0 { "msg.welcome_humans" } else if humans { "msg.welcome_both" } else { "msg.welcome_rivals" }, vec![rivals.to_string()]);
+        g.add_wilds(seed, true);
         g.init_research(None);
         g.init_fog();
         g.place_dwarf_caves(seed);