feat(citygen): params validation, GenError, seeded rng helpers

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/citygen/src/lib.rs

@@ -4,7 +4,9 @@ pub mod geom;
 pub mod ids;
 pub mod net;
 pub mod params;
+pub mod rng;
 pub mod tile;
+pub mod validate;
 pub mod world;
 
 pub use geom::{Dir, Grid, Pos, Rect, Size};
@@ -12,4 +14,5 @@ pub use ids::*;
 pub use net::*;
 pub use params::*;
 pub use tile::*;
+pub use validate::*;
 pub use world::*;