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/main.rs

@@ -30,6 +30,7 @@ mod sound;
 mod spells;
 mod touch;
 mod ui;
+mod wilds;
 mod xm;
 
 use cloud::{Cloud, CloudEvent};